That's strange. Usually you shouldn't see JustMock in the Extension Manager because it's installed only as an add-in. Btw, do you mean JustCode or JustMock? Currently only JustCode is installed as an extension in VS (hence appearing in the Extension manager) and JustMock is installed only as an add-in.
In Order to disable JustMock completely you could go to Tools -> Add-in Manager and uncheck the checkboxes appearing next to JustMock. E.g. you could uncheck the "Startup" checkbox appearing next to JustMock, then restart VS and JustMock won't get loaded.
Hope this information helps.
All the best,
the Telerik team