IMDB are loath to delete a credit, as the assumption is someone must have played the role, even if the attribution to you is incorrect.
So you have two choices, in the order that'd be most helpful to the database:
* You can research the particular piece, find out who actually had the role and edit the credit so it points to this other person.
* You can split the wrong credits off to another entry of the same name, in the hope that someone else will come along in the future and sort it out.
Unfortunately, you can't rely on the latter so it'd be best to fix it right first time and that might be more useful to you, as it'd stop editors in the future trying to merge the credits back into your entry (although you still have to be vigilant for it).