"C. DALE PETERSEN. suffocated a black mountain grizzly bear by arm in the throat and used his teeth that grapples a jugular on the grizzly neck. The grizzly fainted and he finished it off with a stick by striking repeatedly at the head. That's one of the 2 stories. The other was seen a man and a grizzly bear in the pit, deep in the woods that both were killed. No body knows what happen but that bear was killed by bare hands as well."
Both instances are more or less unverified and thus qualify more as STORIES than actual proof. If we choose to believe the first story, then by all means none of what Petersen did to the Grizzly would apply while fighting an Alien as it has low-caliber bullet-proof carapace, non-terrestrial physiology and acid for blood.
For F sake, the creature survives the vacuum of space, spaceship jet blasts, hot lead baths, can punch through thick bullet proof glass and bulkheads. I don't think you can kill an Alien with blunt force - at tops you'll stun it; the creature often seem to use its head as a ram when breaking through doors etc. so with that in mind why would a punch, kick or knee do anything to an Alien? Sure, pressure-wise, on a blunt force kind of way, you could technically try to crush it, but NO human possesses the weight and strength needed to do that (unless using a Powerloader, machinery or a vehicle of some sort). Also, in order to crush it you need to hurt it badly or pin it down first (see Wolf in AVP:R, the APC in ALIENS or the barge crane in A:C). So in other words, no.
As a side note, watch Celtic melee Grid, swinging him around and bashing him headfirst into stone pillars, and Grid just shook his head a little and pounced back at Celtic in no time. No concussion effects or inner injuries suggested, in fact Grid is just as vicious as before despite being mutilated and scarr-tattooed by Celtics before killing him. Sure, we have Wolf manhandling Aliens in AVP:R, but that movie had no consistency and portraid the Aliens as annoyingly incapable, dumb, slow and weak. Now, pit Wolf against Celtic and tell me who you think would win in an unarmed grapple/wrestling fight, or pit the AVP Aliens against the AVP:R Aliens and tell me who would win (I'm actually convinced that Grid would take care of the AVP:R Predalien with ease).
Sorry for the detour, but there just is no scenario or setup where even the strongest most able human would be able to kill an Alien in melee bare handed.