The barmace is short. Use a longer weapon, the stab ability, or a ranged weapon to ward off the overhead smash. Polearms should have zero trouble with this, one-handers can stab (except picks, maces), and most comparable 2H are longer. Also, overhead smashes are easier to dodge.
The bar mace isn't THAT short. Certainly a polearm or longish 2H can get a swing (or maybe even two) in before the bar mace user is in range, but it's not difficult to block a single strike (plus a single strike won't kill someone). Unless you're a ninja (or other suitably fast build) you won't be able to run backwards fast enough to continually back-pedal and swing out of their range.
The issue is that the bar mace is too fast for a crush-through weapon - hell, it's faster than most of the high-tier polearms! An overhead from a Great Maul is easy enough to spot and swing at so that the Maul user is hit and their strike interrupted - the bar mace, on the other hand, has a faster overhead animation than many 2H and polearm weapons (so there's no way in hell you could get a swing in after having spotted it). Again, unless you've got very high athletics, you can't backpedal quickly after spotting the overhead, and strafing does nothing (I've seen many people killed by a 'loloverhead' where the bar mace user does an overhead and spins around, killing whoever is near them with the final moments of the animation).
Also, this is coming from a bar mace user (granted, my level 22 bar mace character is nowhere near as awesome as the other bar mace guy on our - the Australian - server).
Edit: Braeden, the guy on the Aus server that crushes through frequently has 24 strength and 15 agility. Certainly other builds have a little more agi (18 or even 21), but 3 agi isn't enough to say "I'm so much faster than him that he'll never get a hit in!". It's got frigging 93 speed, so 15 agi + 93 speed will often hit you (even if it's a little less frequently than you hit them), plus it does enough damage that many are out of the fight after 1 or 2 hits.