Well, not really a way to "escape" melee, but more of a deterrent. I've been thinking about a way to make hellfire better, and I came up with an interesting idea. Completely change the way the spell works, add a 3 or so minute cooldown, and have it apply a 15 second buff to the warlock.
I'm thinking something like, the warlock becomes engulfed in flames for 15 seconds. Each time the warlock takes physical damage, 20% is reflected back to the attacker as fire damage over 5 seconds, stacks 5 times. If the warlock dies while under the effect of hellfire, the warlock will explode causing all enemies within 10 yards to become pacified for 5 seconds.
The idea behind this is to get melee to stop attacking you. You get a slight damage reduction, they receive a good deal damage and if they do kill you, they are unable to attack for 5 seconds. It's not ridiculously OP, because the reflected damage wouldn't be much at all. Melee hits for ~5k damage, 20% returned would only be 1k damage over 5 seconds. 5 stacks of that would only be 5k damage over 5 seconds, which is isn't that bad and can be completely avoided by not attacking.