You have to be an Event Manager, Team Manager or System Manager to remove a volunteer from roster.

To remove a volunteer from an event roster, take the following steps: 
1. Click on the Manage tab and locate the event 
2. Click on the event name and you will land on the roster page 
3. Click on the check box next to the volunteer(s) that you would like to remove 
4. Change the option on the drop down at the bottom of the page from Contact to Remove 
5. Click the Submit button 

This will remove the requested user(s) from the event roster.