Car Seat Issue.

How about a pop on the butt every time she does it?

No, there is no product you can use. What youcan do is take the rough side of velcro (get the sticky back kind) and place it on the buckle where you have to press to get it undone. It will be rough enough where she will probably leave it alone.

And discipline. Make sure the harness is tight (she should still be rear facing.... per new recommendations by AAP, NHTSA and all car seat manufacturers), so the harness should be at or below her shoulders, and snug as a hug, chest clip level with her armpits.

Have you called Britax? I'm not sure if there is anything out there to solve the issue, but generally speaking you shouldn't use anything with a car seat not specifically designed by the company that made the seat, since outside products may not have been tested on that specific seat.

I would buy a set of the strap covers for car seats. They sell them at bbrus. Cut it to size & clip the buckle inside it. That should make it very difficult for her to unclip. It will probably be harder for you too, though, it would be worth it if it keeps her strapped in.
As for discipline...I don't see how you could appropriately discipline her in the moment. By the time you notice she is unstrapped & you safely pull over, the moment has passed. I think your best bet is to repeatedly explain that she needs to be strapped in for safety. Maybe you could suggest she play with the straps in her stroller. Or, (i have no idea if they actually make these) buy a little doll carseat & have her strap her doll in before you strap her in. And have her tell her doll to stay in her seat for the whole ride. Then after you unstrap her, she can unstrap her doll. If she gets to be in charge of someone's straps, maybe she will leave hers alone.
BTW my older son did this a few times. I told him to reclip himself & he did. I did not need to pull over. I did not get mad at him. I disagree with the other posters. I don't think this is a discipline issue. My son had been clipping his top clip for a while & he was excited that he figured out how to unclip it. He was not trying to be bad & he didn't know it was unsafe. He was exploring his environment. I asked him to reclip. He was able to do it & we continued on our way. I explained to him that it was unsafe to unclip while driving. I also told him I would let him unclip when we were getting out of the car & mommy said it was ok. He did it a few more times & then he was done with it.
Good luck.

I agree that this is a discipline issue. However, what about a rubberband or piece of duct tape over it? If she can't get it off, she'll give up. But she is old enough for a serious consequence for this one-- whatever that is in your house.

My son did that once or twice, but I aksed on here and people said it was because I didn't have the chest piece in the right spot. It's supposed to be aligned with his armpits. I started making sure it was there, and low and behold he hasn't done it since.

If it is up on the chest where it is supposed to be, and tight enough (think one finger, I read a few weeks ago somewhere that the two finger test is no longer recommended) it shouldn't be that easy for her to unbuckle it.

My daughter will be 2 in June and never has tried to unbuckle herself. We did have a problem with my 3 year old for a few weeks when he was 2 though.

You need to make sure without a doubt that she knows that is NOT okay- I don't believe in spanking but I think this is the exception. This is her safety, she could be seriously injured or killed if you were to be in a wreck.

ETA- This is not meant to come out snotty, so please don't take it the wrong way. Like I said my daughter will be 2 in June too, she is also 35 inches tall and wears a 3T. Being bigger and stronger has nothing to do with it, it's a dicipline issue at this point if she is almost two she -knows- better. You need to get a handle on it before she starts climbing out of the seat while you are driving.

Hmm, I have always had britax car seats and my almost 6 year old has just now learned to open that. I have difficulty opening it, and have used them on the decathelon, the marathon, and the boulevard, and now the frontier. the frontier is the easiest to undo, so I am wondering if you got a loose one. I would call britax and see what they say, maybe they will replace the part for you. Once my daughter figured it out, I did have to tell her it was not safe to do while driving and only when the car is parked and turned off and I say it is okay and then she does it. But I really don't think a 2 year old should be able to undo it. Like I said, I had difficulty undoing them, and my kids are also big and strong for their ages, my 3 yo is wearing size 5 clothing and he is huge and incredibly strong, but he has trouble still undoing it himself. It really sounds to me like it is a defect. Unless I somehow got a bunch of carseats with super tight fits, but we had a car accident 2 years ago and had gotten two new seats out of that, So I have had a total of 8 britax seats all of the chest clips very difficult to undo. Like I said the Frontier booster has the easiest to undo and I am guessing that is because it is made for an older child. That really sounds unsafe. Call Britax first they may be able to help. Good Luck.