You DID take their clients away from them. So, they lashed out at you with a dirty trick. This does not justify forging documents, but in their eyes, you are a cock sucker.
Yes, they screwed up by not being able to keep their customers - however, you know the type of scumbags you are dealing with here.
Move on and avoid them as best as possible.
Now, if you want to get cute, send them a professional photo of yourself with your signature reading; "You might as well have an authentic signature - here you go! All my best, XOXOXO. PS, next time you forge my signature, dot the I's with little hearts" - just be sure to dot your I's with little hearts.