How to Block/Unblock a User in Joomla
If you were to simply delete a user they could re-create the account using the same email address. Blocking will at least block that email address from accessing your Joomla site. 

How to Block a User in Joomla

1. Login to the back end of Joomla. If you need help logging into your back end see: How do I access the back end of my Joomla site?
2. Go to the Users Menu > User Manager.
3. Select the user you wish to block.
4. Under Account Details look for Block this User and change this to yes.

How to Unblock a User in Joomla

1. Login to the back end of Joomla.
2. Go to the Users Menu > User Manager.
3. Select the user you wish to unblock.
4. Under Account Details look for Block this User and change this to no.