I don't know the specifics, but I'm sure you could get statistics about the behavior of players that cheat this way--in other words, find things that the cheaters have in common (length of turns, consistency, etc) and have your application automatically find those and put a "red flag" on players that look suspicious. Then you can personally review them (or have someone else do it) and see if indeed they look suspicious. If so, ban them.
Other than that, there is really not much you can do unfortunately. The above suggestion is a lot of work, so unless you're willing to put in the hours to create such a system, I wouldn't even bother with it. Whatever barriers you put up, determined cheaters will get around them.
