Mine eat till they get full then either kill and leave them or just Dont bother with them.
I worried about the same thing but found that they Do stop when their full and content.
It's just like any critter I guess.
A tarantula will not eat to the point to the bursting or otherwise injuring itself. Most will either refuse food, or even kill and leave it as was described. Some will eat to tarantula obesity though.
There are differing opinions on wheter T obesity is bad or not. I tend to think it's relatively harmless in captive situations but I don't feed mine if they're sufficiently plump anyhow - the T doesn't need it, it does carry some risks, and it save me money.