I have this router. It's an excellent little piece of gear.
You need to add an additional rule to the WAN filters. Stick it in the filter set with the TELNET/FTP/HTTP rules. Find out what port IMCP ping uses (I can't remember offhand) than make another rule to drop all packets requesting on that port.
Just use one of the other rules as a "template" just write down all the settings and essentially duplicate it, but change the port number, of course.
Hope this helps.