Doka allows to block/unblock the users in the User profile transaction (DBxUSR),

Why to block the users: 

The relevant user is blocked, for instance., if a user leaves the bank or the department and is no longer to have access to the application. If a wrong password has been entered into the system (say maximum 3 attempts), the system automatically sets the status of the user to Blocked. The system administrator either can Reset the user (with new password set by the system administrator) or set the status to Expired or to New Password.

To block the selected user, click on Block function icon and change the user status to Blocked and then Save it,

Now the user status will be moved to 'Blocked' thus not allowing the user to login into the system,

UNBLOCK:

To Unblock the user, click on Unblock function icon and change the user status to Normal and then Save it. This will unblock the selected user,

Now the user status will be moved to 'Normal' thus allowing the user to re-login into the system,

To set up new users refer article, Set up New Users