Hi, K.. Hmmmm...I've wracked my brains a little bit to try to think of something that might help. The only thing I can think of is (1) your daughter might be spitting for a physical reason such as post nasal drip, which tastes really nasty. This may be why time-outs have not made her stop, because she really, really wants to get a bad taste out of her mouth. It could be any bad taste. Maybe she put her fingers into her mouth after touching something that tastes bad, or perhaps got a little piece of something in her mouth that she doesn't want to have in her mouth, and she really needs to spit it out. If this is the case...then you need to give her an alternative to spitting on the floor or wherever. My suggestion would be to give her a hanky or a paper towel to have on her, and then train her to spit only into that...sort of like what she's supposed to do when she sneezes or blows her nose. She can be taught to either give the used paper towel to Mom or throw it in a garbage can. With a lot of positive reinforcement, plus having you model the behavior for her, too, she will actually want to stop spitting on the floor.
(2) Sometimes children will engage in a power struggle over things that parents find important. If this is the case, I would still give her an alternative place to spit, rather than trying to wipe out the behavior altogether. That would lead her away from the spitting behavior in small steps until she stops altogether...but don't try to make her stop altogether without re-training her.
Try to find out if someone else around her is spitting, and it's only your daughter who gets caught. This will make a small child FURIOUS if others are allowed to do something, but they are excluded. She may have seen little boys (or grown men) spitting on the ground outside without anyone complaining, and then she doesn't understand the huge difference between the ground and the floor of the house.
Even though little girls usually identify very strongly with their Moms and are extremely eager to copy female behaviors, they can also be influenced by men or boys, and can be extremely upset when boys get rights or priveleges that girls can't have.
I know that spitting is pretty distasteful, but try to keep it from becoming a big, angry issue between the two of you. If she has some developmental issues, she may not be able to express herself or articulate what she's feeling, so she does a physical thing which relieves whatever is causing her to spit. A hanky or paper towel alternative may be the only way to get her to stop using the floor until you find out what's causing the behavior.
Anyway, I hope this is helpful to you.
Peace,
Syl