Get a door camera for your gran, something simple you can also control over an app for her, like a Ring camera, and let him know she has it. Once he knows the camera watches he will probably be a tad less recless if he is not completely retarded. It's also good to have that footage if need be. Also as others said, get her a lawyer and if you get footage of him harassing your gran get a restraining order so he can't step on her property.. that will make things a lot easier and usually doesn't take that long.
Other than that i don't think that petty pranks will help ... better to get him on legal grounds bc that has actual real consequences for him.
Buuuuut, and that it completely hypothetical and not a real advice of course - just silly thoughts - , if you really wanted to get to him, you'd sign him up for all sorts of trial subscriptions and shit, but careful it can't be traced to you. If you had some cash to burn order things from shitty pages like temu, but only if you can absolutely securely pay without trace, like with cash-payed gift cards and shit.
And of course never ever ever talk to anyone about that ... ever.