Deleting contacts should be the last course of action because deleting means you lose history and reports that include deleted contacts won't be accurate anymore.
Invalid and unsubscribe contacts don't count towards your total contact count and as such they aren't used for billing.
When contacts are deleted you lose all history such as form submissions, email opens, clicks and consent details.
Instead of deleting contacts we recommend that you to mark them with "revoked" consent state - that way history is kept and the contact will not receive any emails going forward. (how to bulk update consent status)
How to delete contacts?
You can delete contacts from the contacts list page: Click the Group edit button and select the Delete contacts option.
Note that this operation is limited to deleting contacts appearing on a single page.
To bulk delete all contacts please contact support.
Some contacts cannot be deleted
Contacts with consent status "revoked" or "spam reported" cannot be deleted. This restriction is in place to prevent these contacts from being accidentally re-added to the database as subscribed contacts, which could create spam complaints and compliance issues.