campermom, I did not realize you were local to me. I am so sorry your ex is such a raging asshole. I am guessing he only has issues when spending money on others and not himself.
Like pp said all you can control is your reaction.
I have an iPhone and silence all my alerts and notifications. I don’t need to see most of it at all, let alone instantly. The only thing he needs to know from me and is legally entitled to know are kid logistics and unless they’re with him, there’s a 100% chance the message can wait a few hours and a 99% chance it can wait 24 hours. His sending messages and requests does not entitle him to an immediate recognition or response or even any response.
I’d totally ignore the Venmo. If he does request and get pissy, tell him he can discuss it further with his attorney as the matter is settled per your divorce decree and use the language in it if he persists.