My first and best advice is not to waste your time. If there is a warp in it now, pretty much no matter what you do there will always be a warp. If you take it out, it will come back. Of course there are few exceptions, but don't count on it.
If you are going to waste your time, and if the warp is very minor, you can hang it in your closet. Wrap a rubber band around the tip/ferrule and let the cue hang for a few months with a weight attached to the other end of the shaft. Make sure the shaft is hanging straight up and down. Hang as much weight as your contraption will hold.
Another really stupid method (that I have tried before) is to find the pinnacle of the warp and hold that place against your knee and bend the ends of the shaft inwards against the warp. This will either solve your problem for ten minutes or it will break the shaft (probably the best outcome for a warped shaft). IF the shaft is way too warped, cut some notches in the tip/ferrule, and in the joint, tie some string to each and pull it tightly...then go buy some arrows and take up archery.
IF the cue is custom made, call the cue maker and ask for a replacement shaft.
Deno Andrews