When you say remove, do you mean from the database or is from a drip campaign where they are in a wait step? Regardless, you will need to create a smart list using the "Email Bounced" filter. There you can reference the email you sent and a list of bounced emails will appear. To be clear, these are hard bounces. These are the ones that Marketo feels like the email is not deliverable too. You can then select all and Delete if that is what you are looking to do. Or you can select all and Remove from Flow to remove them from a drip campaign.
If it is a drip campaign, you can always set up a trigger campaign to remove them from the flow if an email bounces. This will eliminate the manual work later on.
I wouldn't recommend changing the email invalid field. While Mkto is not perfect, that is usually a pretty good indication of whether the Lead has left the company, or changed their email, or generally will never accept an email from your domain/IP. But if you do want to: select all, Change Data Value, Email Invalid, false.