Search via the left margin for their email or user account to bring up their user account.
There should be an Edit link to the right of their name. Click this to bring up the Edit User form.
For User Level, change it to Pending Author to give them access to the forum. Change it to Full Author to give them the ability to go live with blog posts.
To ban a user
If a user is posting blatant spam (eg - viagra products usually posted by an automatic bot), you need to ban their user account asap and delete all comments. Do this in one go by clicking the SPAM button beneath any of their comments. This instantly moderates all comments by that user and sets their User Level to Spammer.
However, you may wish to ban a user without deleting all their posts (eg - a troll who has outworn their welcome but deleting all their comments would sabotage many old threads). In this case, search via the left margin for their email or user account to bring up their user account.
Click the Edit link to the right of their name. Click this to bring up the Edit User form.
Set User Level to Spammer. This prevents them from logging in or posting any comments.