So I'm assuming you have a similar setup to this guy:
and sadly his solution was to install an additional mikrotik to just filter udp traffic to prevent exactly what you're experiencing.
You're pretty much the victim of a DOS attack unless you can figure out the true source as a virus or somesuch.
And just to be sure, firewall rule on rate limit should be on 'forward' chain if passing through router, 'input' if directed at the router. Might want to try both and see which is more effective.
Does traffic stop when you stop forwarding port and drop all UDP packets directed at that port?