Edit: I found a much better script, perfect for your problem.
Auto-Signout made by SpunkyLoveMuff.
Download and import the script. Then you have to go to Script Preferences and edit the script. Enter your e-mail address where the banned@email.com is and save it. Once you sign-in, you will have 60 minutes on WLM.
You can see the timer when you open a conversation.
Hope it works!
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The best I can find is Monitor Lock 1.0. When you lock WLM, the monitor turns off. Hopefully that will keep you away from WLM 
To change the shortcut to lock WLM, go to Plus! > Preferences > Messenger > Messenger Lock...
Then make a shortcut to lock WLM and change the settings to what you like.
Once you've been on WLM for about an hour, do the shortcut to lock WLM and your monitor will turn off which will hopefully not tempt you to go back online To unlock, do the shortcut again.
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