If you delete someone they can come back with the same information - but if you ban them nobody can join your nation with those credentials again.

From our documentation:

Deleting a person: Removes her profile from your nation and abandons any user-produced content. Deleting a person is irreversible. A group of people can be deleted using a list. 

A profile cannot be deleted if the profile has:

• a permission level (i.e. control panel access)
• made a donation
• been sent an invoice
• been connected to an expenditure
  • received an expenditure
  • created an expenditure
  • listed as the person / organization the expenditure was about (in support of or in opposition to)

Banning a person: Profile is marked as banned. No one can login to your nation using the Twitter, Facebook, or email accounts connected to the profile. Once you ban a profile with a Twitter account, you will also block them on Twitter. The person can be unbanned. This process is usually done after a person receives three rules violations.