How to remove a crank arm with stripped threads?

Your crank arm is trashed.

With the crank bolt removed, gently ride around a few miles, it should work itself loose enough that you can yank it off.

Otherwise many shops have basically a slightly larger crank puller for addressing this issue, where they chase out a larger set of threads, then use the larger puller to get it off. The crank arm typically goes into the garbage at that point however.

A similar idea to whatsisname's: Use a rubber mallet to tap on both sides of the crank arm, alternating between the inside & outside, so as to work the arm loose. The hits do not have to be brutal, but with enough force to get some movement happening so that the arm is loosened. Put more force on inside face, since you are trying to take the arm off, and only a light tap on the outside face. This method has the advantage of not having to get on the bike and risk an accident should the arm come off unexpectedly.

http://www.parktool.com/blog/repair-help/removal-of-cranks-with-damaged-threads-square-type-only

http://www.ehow.com/way_5865105_bicycle-crank-removal-stripped-threads.html

Bicycle Research Crank Arm Thread Chaser, TC-8

or:

you can try a tool called the "Var #932"

If you know someone that works on automobiles ask if they have a tie rod remover.It looks like a heavy duty tuning fork with tapered ends.In the absence of one of these use two large screwdriver and wedge them between the crank arm and the bottom bracket,being careful not to damage the chainring.Tap the screwdrivers or the tie rod tool just hard enough to exert some pressure.If it doesn't pop off try to warm the crank arm with a heat gun or a hair dryer.The idea is to heat and expand the crank arm but not the shaft. As it warms tap the scewdrivers to see if it moves.You might want to apply some lubricant just be careful of heat and combustable fluids.Let it sit over night with the pressure applied and keep tapping the tool until it releases.You can also try to tap the arm with a ballpeen hammer while holding a second hammer 180 degrees opposite.The shock from the moving hammer distorts the crank arm hole while holding the second hammer stabilizes the shaft.This seems like it involves some complicated positioning and may be easier with two people until you get the hang of it

Thought: Go to a building supply place and get a bundle of tapered shims. Probably the plastic ones would be best. Insert four shims, in pairs, with opposing tapers, one pair on each side of the crank shaft. Tap the shims in tighter and tighter until the arm pops off (or the bearing cup pops off, whichever occurs first).

I've just been through this in a restoration I'm doing. I have a decently equipped shop so some people might have to find a friendly mechanic... The Park tool simply pulled the threads right out of the crank. I've not had that problem before so I know it's not me. I have often suspected that the Park tool is very slightly undersized, so this did not inspire my confidence.
I tried heat (MAP gas, don't use oxyacetylene, it will melt the crank), various solvents/releasing fluids, riding the bike without bolts up steep hills, tapping with hammers on the sides etc... all to no avail.
Finally before giving up and cutting the damned things off, I took it to my friendly bike shop where their profession-grade tool was able to dig into the remaining threadforms and just barefly get the drive side crank off...but not the other. From here I could disassemble the bottom bracket and used a 12 ton press to push the axle out of the right crank arm. The taper LOOKS OK, but before re-installing, I'm setting up to bore out the crank arms and push in a steel threaded insert. I'm going to try retaining the insert with just friction and Locktite Green (671) but it may need to be peened. Doing it this way shouldn't weaken the crank arm and will provide steel threads to pull against next time.... Not the answer for everybody, and it may be too costly to hire a machinest. Even with the tools and ability, it would probably be cheaper on some scale to just buy a new crankset, but then why replace when I can fix it?

I just heated the crank arm up with a butane torch, grabbed an aluminium drift, laid by bike across my knees, and gave the boss where the inner chainring bolts on a firm blow with a hammer using the drift. Came off real easy.