And yes, it is indeed annoying to receive such invitation if you don't play the game, even though the invitation only appears in your notification and not on your wall.
Instead of blocking your friends, here's an alternative:
Step 1:
On your FB main wall, look at the left panel, under 'Apps', choose 'Games'
Step 2-4:
Under the tab 'Activity' >>> 'Invitations', you can look at the list of game invitations you received.
Choose the one you wanna block (and in this case, its the Pirate Kings)
Step 5:
After clicking the 'X'(step 4), FB will be asking you for confirmation.
From there, you can choose 'Block Pirate Kings' and you won't receive any invitations (or receive any notifications) from that apps anymore.
Step 6 is optional,you can choose to ignore invitation from that friend from now on. :D
