What should a firewall rule destination say to block packets sent to address 239.255.255.250?

My firewall passes packets being sent through one of its interfaces, to the destination address 239.255.255.250. I want to create a rule that will stop these specific packets, but when I enter 239.255.255.250 in destination, then the packets keep coming. The rule requires netmask for destination addresses, and I tried entering 32, 24 and other values to no avail. Not sure if it’s netmask that is failing the rule or something else. If I switch the rule to block all destination addresses, then it works, so the rest of it is correct (source, port, protocol, etc.)