Another idea : the use of a captcha.
URL shorteners are full of spam links created by spammers robots.
A captcha would help to stop them.

A test could be done on the visitor IP and if there are too much URL created in a short amount of time, the IP would be blacklisted
The SPAM is a real problem with these URL Shortener

Thank you again for your good work

>Another idea : the use of a captcha.

That's a great idea, too. Thank you for your feedback!