How to Keep footer link remain intact?

If you are releasing a GPL’d theme (which WP requires, but not looking to start the whole GPL debate here) then you can’t prevent someone from removing your footer link. You can ask they they don’t do it. But anyone who’s looking to do so will get around whatever method you employ.

a side note: having these links in the footer may be more of a bad thing than good. I’d suggest reading this article on WPMU about how spammers using their themes caused them to get thrashed in their rankings.