I have a bantam cochin who does the same thing. The only thing that stops her is to actually let the little bugger set awhile. A one point, after her fourth attempt in five weeks, I let her set for a bit, then slipped three new babies under her. That finally stopped the cycle ... for THAT year, anyway. The issue is that you either need to have space for the new birds yourself or have a home ready to go for them. Sometimes, Chicken Math is just the easier way to go ... sigh ...