If they are sharing an account, they aren't really cheating - your application just isn't providing them any benefits to have multiple users.
If you find a large percentage of accounts are shared, then maybe you should take a look at revising your pricing and segmenting on something else. Alternatively, you could look at adding functionality that makes having multiple user accounts more useful than just sharing one.
If it is just a few accounts, it's probably not worth your time worrying about it.