How to add free domain extension in spam list?

Hello, I am receiving a lot of spams from emails which are using free domain. I am working only with companies if they are using paid domain. So I want to add free domains (.tk, .ml, .gf .....) extensions into SpamAssassin Spam List. Is there a way to do that? I know how to block all addresses with one domain, example: *@example.tk But I want something like this: @.tk