It just dawned on me that I could use my ex's email address when signing up for websites so that I can avoid spam mail.
Oh dear. Do you really think using your ex's email address is a good idea?
Yes! I don't want spam mail and I also want to get back at him. This is perfect!
It's understandable to upset at an ex, but intentionally sending them spam emails is not good. They will probably figure out it's you that is causing it and will confront you about it. Instead of using your ex's email, just make a new throwaway email to use instead.
I don't care if he finds out.
Aren't you worried about what he will do when he finds out it's you causing all the spam messages? He is liable to do the same thing to you or something worse. It's best not to agitate him and instead move on.