Can't sign out of messenger

Someone used my pc months ago, and unknown to me signed in to messenger.
Well I've never used the **!!** thing before and I have no intention of
doing so.
My problem is it keeps popping up with people trying to contact the sole
previous user.
I've tried close, exit and still it keeps coming back.
Now I've uninstalled the damn thing but still people are popping up but with
their email address followed by "conversation"
How the hell can I sign out?
I can't start a conversation with the callers as it's in Thai and I have no
idea (although my XP is English).
Any help please?
TIA
Ewan


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