I don't know if possible or not but i don't think that should have anti-surrogacy laws simply because one mother probably can fulfill another mother's dreams. Lets say a couple who can't have child at all, even with all the medication help or any other ways, surrogate mother can, if willing can help someone live mentally balanced life.
No, it is not impossible to enforce anti-surrogacy laws, because the laws help a lot of people. With DNA testing, it is easy to avoid false complaints and complaints of fraud. DNA testing can be employed if there ever is a question. People who enter into these contracts should be fully informed, but should be able to enter into the contracts for the benefit of everyone.