Just don't log in for 6 or 7 weeks and you'll be made inactive. My advice, though, is that if you want to come back, keep your team alive. All you have to do is to log in once a month or so. Don't do anything more and you'll be able to come back with a nice pot of money. If you do a little more like setting your training right, you can wind up with some good players.
Either way, you'll be much better off than starting over again.