I had a crepe myrtle cut down at least 6 years ago and I still have suckers trying to come to life.
Is there anything I can put on them to kill the root that seems to still be alive?
The easiest way would have been to apply a very small amount of concentrated (the stuff that's labeled something like 35% or 40%, not the "ready to use" stuff) glyphosate or concentrated triclopyr to the phloem area of the freshly cut stump just after you cut your crApe myrtle down. That would have eliminated the problem to start with, and would almost surely have taken care of it with just a single treatment. By the way, glyphosate is the main active ingredient in most RoundUp products and is also available in other brands or even in generic versions. The "regular" RoundUp formulations generally work a little better for this application than the fast-acting stuff. Triclopyr is sold under brand names like Brush-B-Gon, Garlon, and Release.
Anyway, now that it's too late for that...get a applicator bottle (an adjustable tip mustard bottle, a contact lens solution bottle, or something like that) that will allow you to dispense just a drop or two at a time of the above mentioned chemicals. Clean that bottle very well and put a small amount of the concentrated herbicide in it. Label very well to avoid potential problems (even if you aren't going to store the chemical in the temporary dispenser). When you see a sprout shoot up, cut the sprout at ground level and immediately apply one or two drops of the herbicide to the freshly cut "stump"/shoot-base. Some shoots may be killed with just one such treatment, others might take a couple of treatments. But, it won't be long at all before your unwanted crApe myrtle sucker problem is no more.
BTW, if you have more questions like this, you might try the Trees Forum, Shrubs Forum, or some other related forum. If you want to grow more crApe myrtles, ask here in this forum!