Unfortunately, if they have been once, the chances of them coming back again are quite high because they know you have bikes, know where they are and know how to get at them.
There are a whole host of things you could do from simple to overkill.
The simpler ones , as mentioned, are to keep them locked up at all times. If they have to be outside rather than inside, and if youhave quick release wheels, take a wheel off each - they will not be able to ride them away and would make them difficult to sell on. If appropriate you could go for ouside security light and also go for outside camera - even a dummy would do.