I have been struggling with this problem myself, and honestly, its quite annoying! When this is happening to me, I do three things:
1) Remember, he's getting just as aweful hands as you are!! He doesn't have the nuts every single time so try going all in first(if you have a hand to do it with!!) and see what his reaction is. If they're that aweful, they're gonna call but show some aggression towards these players. It seems to slow them down when I do this.
2) The blinds start out low, so WAIT! Don't wait so long that youre up to 30/60, 40/80. 50/100 on the blinds, cause by then you stack is gonna be short and that's no bueno. There's nothing wrong with being a patient player against these types of people. Honestly, I prob wouldn'tve shoved with A6. I'd wait around for a hand I felt more comfort with, such as a pocket pair(77 and up), Ace 9 and up, suited cards.. my point is, I would wait for a hand I felt COMFORTABLE with. Not get aggitated and be like Ax, screw it.. im sick of this, I call. Cause honestly, if you dont feel comfortable w/ a hand, why play it? Esp against a donk of this magnitude lol and last but not lease
3) Realize, it is poker... bad beats will happen! So if it doesn't, shake it off and go from there. Don't let these donks take the enjoyment out of it for ya! Maybe even put dudes name on a list so you know not to go up against him again cause its just gonna be random all-in calls with 2-7, and I say this from experience =P I had QQ and was like "perfect, I call!" Flop 2-Q-7... three of a kind! woo hoo! Turn 10... you can guess what happened on the river... and no, I did not win that hand.
But that's how I play those kind of players... I wait for a DECENT hand that I would play in an early position if I was playing a cash game, or MTT close to the bubble lol So that takes my range of hands down, but at least I feel comfortable when I do call... I dont think widening your range here is a good idea due to the fact that he's gonna call no matter what he has, or what he thinks you have so the old saying "patience is a virture" comes in handy here... but then again, once can't be to patient