You want to know if there are any issues with deleting a user and if anything tied that account will also be deleted.
Deleting a user will not affect any assets in your Marketo instance. All of the assets they created will remain in your instance and you will still be able to see what edits they have made.
If you want to keep the user but cut off access, you can assign them a blank role with no access permissions. That way if they do log in, they won't be able to access anything. Assigning an "Inactive User" Role to all users you want to remove access for can also help you to quickly see who once had access in your instance.