actually such a thing takes maybe 1 hour to do... rather simple grab ip when user makes the url (or when they request it) post it set the expiry time (time+6 seconds or what ever you chose and depending how you do it it might be best if you use the time thingy SMF uses more accurate), when the user requests again take the ip check the time and IP against the ones in your DB (using IP as reference) if its before the expiry then kill process stating what you want.
Note: to save precious time and space and crap set up a cron job or something that allows you to trim the expired IPs ever so often.
Advance: if continuously requesting too fast after 5 times Auto 1 hour ban (really not hard either simple $C+1 (C being count and starts at 1 upon first and only good for that duration))
Or could set up a new table strictly for IPs with a time length and ban anyone who has done X number of links with in a time frame.
i think all can be done with in 1-2 hours...