This is a phase that will pass, but will never completely disappear, especially with boys. Set boundaries...let them know that you don't think it is funny and that it is never to leave the house, that if someone gets their feelings hurt it is time to stop and if you say stop they need to listen or there will be consequences. If they begin a round of potty names, ignore it for a bit to see if it will wane...if not, tell them you have had enough.
Try getting involved in their fun by using other types of fun silly names to see if you can transition into a game that is fun that does not involve potty talk.
I am sorry to tell you that fascination with the subjects of poop, farts and butts does not completely go away. Even my boys (9 and 13) will have noise contests to see what sounds "real" trying to gross each other out or they will attempt "underarm farts". They know now NOT to do this when parents/adults are in the room. If I happen to be in the other room, I let them know and tell them I've had enough.
These subjects are talked about by parents all the time too through the potty training years and beyond. Look at the posts here regarding blowouts, removing poop from laundry, constipation, not pooping in diapers, pooping in places other than the toilet, etc., etc. It is an important topic and kids want to be able to talk about it, but don't know how with out just being silly. They learn it from adults, but have not learned when and where this type of talk is appropriate. Read the article Alison suggested. It is great! Good luck! :0)