No opening
Fighter Utility 2
Encounter ✦ martial
Personal
Trigger: An enemy attacks you and has combat advantage against you.
Effect: You don't grant combat advantage for the attack.
No opening 
is a second-level fighter utility power. [PH:78]
It is an ineffective power choice, as it gives what amounts to a -2 to a single inbound attack that expends an expensive immediate action (which is needed to lock down enemies.) The sole use would be against the specific creatures that have an effect when they have combat advantage.[1]
Errata[]
- An unmarked errata changes the wording. It originally said "Cancel the combat advantage you were about to grant to the attack.", which some GMs could interpret as only canelling one source. [Dr398:CC:F]