Is it wrong that I'm thinking of subscribing my partners ex to spam mail?
Subscribing to spam mail is pretty weak in terms of revenge. Why would you want to do that?
I want her to suffer a lot for what she has done.
I think the better thing for you to do is to just let it go. It's something that happens in the past, it is more mature to just move on especially it's your partner's past relationship. Don't let negativity consume you, otherwise, you will just be mad all the time and that's not good for your mental health.
Good thing is I'm not going to lose that much sleep over it.
It seems you are being petty over a situation that may have occurred with your partner's ex. It's best to avoid seeking revenge and instead focus on yourself and your relationship.