My advice is this. If you have a clean save available, back up your hacked save to your computer and reload the clean save to your PS3.
If you don't have a clean save, back up your hack save, then reload GT5, delete/sell all your hacked cars and hope for the best.
OR
Back up then start a brand new save.
Give it a week or two to actually see what the Ban actually does. If it's just for those cheating TT's and Drift seasonals then you should be able to reload your cars.
If it's for all hacked cars, just keep using your clean save.
Also via means I cannot discuss on here, you can change the user of PS3 save games. This being the case, you could set up a new user on your PS3, change your hacked saves user, then have access to your full garage on a different account, which you could then keep offline.