Probably just pay them more to hang around.
According to BBC "A wage cap could also be introduced on the errant clubs to ensure there is no rise on this season's squad wage bill for the Champions League."
To be fair, I was surprised when I read the reports. It's further than I expected the UEFA to go - a squad limit and a wage cap can actually hurt them quite a bit.
We'll see what happens after appeals, etc... but UEFA has taken the matter seriously, imho.