This is my first post on this forum and of course with a question.

I want to block a specific email address from using a form. On my website you can give your email address and a standard email will be send with extra information. Unfortunately someone is using this to spam someone else. Blocking his ip doesn't work because he is using different ip-adresses. The best option is to put the e-mail address on a blacklist, but how do I do that?
Hopefully someone can help me.