Pardot scrubs any hard bounces or opt-outs (unsubscribed prospects) from your mailings.† These prospects will still be on your various lists, but will not receive future emails from Pardot — everything is handled automatically to protect your sending reputation.
If an incorrect email address caused a hard bounce, correcting the email address on the prospect record will clear the hard bounce field. However, the hard bounce will still appear as an activity on the prospect's record. See Pardot Hard Bounced for more information. Note: if you have multiple prospects with the same email address, you'll need to update the incorrect email address on each prospect record.
† If you have multiple prospects with the same email address, Pardot will scrub all bounced or opted-out prospects with that email address.