I have an idea :
We can submit for the same problem immediately after a submission,
but for another problem, we should be waiting for 10 min, it seems a reasonable delay.
Easy problem can be done under 10 min, okay, but we just have to wait a little, it would be very rare.
I'm even ok for 30min of delay, it's not a problem for me.
This can discourage cheaters to copy/paste solutions that can be found on the web,
those guys don't hesitate to submit every 20s an new problem submission.
Moreover, I think, all those accounts should be deleted, they are cheaters !