We just went through something similar... First: Read your house rules. Do you allow sublets?

We are now enforcing our house rules. Which, basically does not allow sublets -- except on a one year basis, for owners with serious needs (temp work transfers)

We are allowing legal sublets to run out and giving notice that they will not be allowed to renew the sublet. Some, that have sublet for years -- are going to be allowed six more months -- BUT ONLY -- if they pay past fees. So far, the prospect of having an empty apartment, trying to sell, as oppose to continueing the sublet -- everyone is paying up, and the ones who are not returning, are putting the apartments on the market.

A healthy coOp is owner occupied.