An admin user would need to login to Altium Designer. From here, access the license management page to see licenses associated with the account. To do this, select the portrait icon in the top right and select Sign-in.
From here, sign-in with your AltiumLive account.
After signing in, select the Licenses... from the portrait icon to access the License Management page. Select the license you are trying to release and expand to get the list of users currently using the license. Select the user you want to release, and right-click ► Release. This would force the user to release the license seat.