You cannot be found guilty of "unlawful" actions which you did not commit, but you may certainly wish to disavow and untangle yourself from this account by contacting the bank. Most banks prefer to close joint accounts and re-open an account for any account holder who wishes to continue banking with them, rather than separating an account holder from the account, which I assume is because the bank feels this is a cleaner cut way of ending any possible future disputes over the account. If you used a lawyer for your separation agreement, you should have that lawyer contact the bank on your behalf to first put them on notice of the date you no longer have an interest in the account and will not be liable for it, plus to see whether they can remove your name from the account; failing that, mail a letter to your ex or his lawyer to get the account closed so your name can be removed (this may be necessary because of the provision in your separation agreement). If there are civil claims or criminal charges pursued due to some wrongdoing by your ex, it is possible that the paper trail leads to the joint account and an effort could be made to collect the loss from both joint account holders, which would involve you having to hire a lawyer at that point to defend, and prove you transferred control to your ex. That is not a prospect I am sure you want to have to deal with at any time, so you are correct in trying to head off any such issue before it happens.
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