Presumably, you have a licensed user list and you audit the use of your intellectual property. I can't help it that this is hard, but that's the only even remotely practical solution that isn't a false sense of security. Wishing that people don't infringe is extremely ineffective, especially when you have an extremely sharp sword, the law, at your disposal to assist with enforcement.

Interestingly, one of the well known cases for keeping a trademark is that you make moves to protect it by stopping infringers. In some respects, it makes sense that those who want other, similar kinds of intellectual property protections be subject to the same requirements. Trademarks and copyrights both protect things that don't have hard physical manifestations that provide inherent limits on their reproducibility, so it may be reasonable that they are both afforded the same methods of protection.

How are you going to hide a @fontface call in your CSS file?

what, and crawl the entire web to find infrigement?

Or pay for someone else to do it - but then, how would you enforce it? The costs of hiring a lawyer to threaten to sue would be greater than the money made from selling the typeface.

It's just a filename. It can be anything.

.. but it'll still be the same file. With checksums and binary diffs, not a big problem. There's even a business opportunity there!