Answer: There's a chemical called napthaleneacetate that's supposedly effective at knocking down this annoying habit. At least one branded spray called Sucker Stopper contains this ingredient. If you can't find it at the garden center, one online vendor is http://www.montereylawngarden.com.
I personally haven't used this spray and can't vouch for its effectiveness. I also get a little nervous about products that supposedly quash one part of a plant's growth without affecting anything else.
I also routinely deal with suckering of my apples, pears and crabapples, but I just cut them off as close to the roots as I can.
Best wishes with your crabapple trees!
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