If you are playing the Mount Doom Gollum fight, it is possible for it to glitch if you are using the Invincibility Red Brick. There's a Fetch-Quest in Edoras (near the Warg Attack mission) where the guy talks about how he used to be a citadel guard, until he took an arrow to the knee. Characters can't swim, but they can hop out of water should they fall in. If you fall into the water, be sure to hop straight back out again. Just make sure the other player isn't signed in with a profile. You can earn all the achievements listed as 'solo' in the description while playing co-op.
IP bans will be reconsidered on a case-by-case basis if you were running a bot and did not understand the consequences, but typically not for spamming, hacking, or other abuse. If you are responsible for one of the above issues.