How do I force active users to log off?

As a Super Admin you can force active users to log off from LastPass so that the master password reset option will become available in the Admin Console.

In the new Admin Console

Log in with your email address and master password to access the new Admin Console at https://admin.lastpass.com.
If prompted, complete steps for multifactor authentication (if it is enabled for your account).
Go to Users Users.
Select your desired user(s).
Select More actions > Destroy all sessions.
Select Destroy sessions to confirm.

Results: Your selected user(s) have been immediately logged out of LastPass from all active sessions.

In the old Admin Console

In the Admin Console, go to Users in the left menu.
Check the box next to "Email" in the user pane to select all users, otherwise you can check the box(es) next to individual user(s).
Click the More actions hyperlink, then select Destroy all sessions for selected users.
Click OK to confirm.

Results: Your selected user(s) have been immediately logged out of LastPass from all active sessions.