just create a filter on the global mail settings, and select the option to discard them. I think the option is 'fail' haven't been using plesk for long, but if you get in there you should be able to figure it out.
You can blacklist the Top Level Domain i.e. tld from Plesk >> Settings >> Spam Filter Settings >> Black List. Just add *@*.ru and click OK.
It has worked for me very well once. BTW, any error message do you receive when placing the tld in Plesk?
I login to plesk > server > mail > blacklist > add new domain (only option available) error I recieve is Error: Domain name *@*.ru is invalid.