I think, many of the more famous CPAN (or other OSS) modules must make this experience. It's not nice and it leads me to add a paragraph to all my maintained modules under which circumstances I will provide free support and that any kind of support can be got via hiring me - looks like other authors had the same idea, see a comment of David Golden: http://www.dagolden.com/index.php/804/expectations-of-volunteers-in-open-source/.

Yes, "Thank you" is sometimes not enough - but you should create the rules (and tell the prices) before you take a job (or before you investigate more effort).

In case of CPAN you can always ask the RT Admins for help if a user is misusing RT (e.g. reopen rejected tickets).

And you might take a look into DBI - there is a good paragraph about "Getting help" - it guides the users perfectly. The users who ignore this will be ignored, too - it's easy.