Kubota L3830GST, B7500HST, BX2660. Formerly: Case 480F LL, David Brown 880UE
I assume that you have a ball between the lower arms of your 3 point hitch on one of those drawbars with a bunch of holes?
If so, what I have done is put a straight clevis (such as: Amazon.com: Koch 4002543 Forged Straight Clevis, 7/8-Inch, Black: Home Improvement ) around the lower 3 point arm on one side (so that it goes around the arm between the drawbar and the tractor) and put the pin through the last hole in the drawbar (this does essentially the same thing as the drawbar lock in Chris's 3rd link, but I can also use it for other things).
That way, it does not rotate.
Also (on a tangential subject), I have tried to standardize on a 2" ball for everything. They are about the same price, but that way there is no chance of putting a 2" trailer on a 1 7/8" ball and losing a trailer going down the road.
Drawbars were meant to rotate, for use with an implement hitch like this:
But people keep wanting to use them with a ball, so all those methods were developed to stop the rotation. For farm use, I prefer the clevis hitch and a rotating drawbar.