This is mostly a Palm OS problem, actually, or at least we haven't been able to find anything in PlecoDict that could possibly cause it; we record whatever points the OS gives us, basically, so if extra lines are showing it up it's because the OS is telling us that the stylus was moved there.
Does this happen every time you draw a character or just occasionally? For occasional problems, the best workaround would be to use the Undo button to get rid of the problem stroke. If it happens too often to simply work around, you might want to call Palm tech support. You could also try hard resetting your Palm (hold down the power button while sticking a paperclip in the reset hole, wiping its memory) and reinstalling PlecoDict after that.