Try and contact support, tell them you didn't know modding the game would result in a permanent online ban and ask for them to unban you. They may just say no but it's worth a try, right?
Hayden, when you go into singleplayer and then try loading online, it will redirect you back to single player. You should see a small notification above the mini-map saying you have a un-ban date e.g 2 weeks, if not you got perma banned