It may be impossible w/ anonymous ones (is it?) but for logged-in users, it would be a simple thing I'd think to stop them from submitting 15-20 pieces of useless dreck / spam in the space of just a few minutes (like this user: http://slashdot.org/~zumed678\). Readers are limited on this basis (comment frequency), and it seems even more appropriate for submitters.
Perhaps in the case of a followup submission ("I forgot to include the link intended -- here's an updated version"), a user should be given the option to have the newer submission replace the older one without posting-limit penalty. ('You made a submission less than 10 minutes ago, titled "Why Men on Mars Should Stay There." Would you like to replace it with this one?')