The only way to beak a 'lock in PvP (world PvP or BG's or arena or whatever) as a Prot Pally is to have the element of surprise. If the Warlock gets off his DoTs and fears you, you are pretty much done, and should probably just eat your death rather than waste pots or cooldowns and save them for another fight.
If you can surprise the Warlock and HoJ followed by seal of Justice and throw your shield to daze him so that he isn't casting constantly and cannot easily get out of melee range, you have a CHANCE (depending on the 'lock's gear, trinkets, etc.) If you can get him down below 20% so you can Wrathhammer him, almost guarantees victory. Warlocks are generally very squishy, but you have to surprise them and do anything you can to interrupt/stun/daze them, or they will fry you like a corndog. Having improved bubble so your melee speed isn't reduced while bubbled helps too, but it is STILL tough.
The easiest way for a Prot Pally to beat a 'lock is to have friends around to help you and hope that the 'lock doesn't. Unfortunate, but very true!