The system doesn't have the option to unassign a user once it gets assigned to a lead. However, you can update the user’s access per campaign level to ‘No.’ Follow these guidelines:
Go to your Profile in the top right corner and select ‘Manage Users.’
Select the user name.
Click ‘Role Assignment.’
Select ‘Append User.’
Select the ‘Roles’ > ‘Market’> ‘Campaign.’ Set the ‘New Leads’ to ‘No.’
Note: If a new lead is generated for that specific campaign, the system will not assign a user to the specific role.