I had some trouble locating how to do this, so I thought I would post this in case someone else is having the same trouble with someone using your account to upvote things.
Just go here: (https://v2.steemconnect.com/revoke/@busy.app)
All you have to do is change the end of this link from busy.app to whatever or whoever has permissions listed on your steemd account on the bottom left of the page under Authorities-posting.
You can check your steemd by going here: http://steemd.com/@your_user_name
Just replace with your user name after the @ in the link above.