Turning off automatic updates is done completely within Visual Studio as there is no functionality within XrmToolkit that performs automatic updates. To turn this off within Visual Studio you can try the following 2 methods:
Go to the 'Tools -> Extensions and Updates' menu within Visual Studio:
Find the 'XrmToolkit' extension under the 'Installed' extensions and de-select the 'Automatically update this extension' checkbox:
To turn off updates for all extensions navigate to the 'Tools -> Options' menu in Visual Studio:
Find the 'Extensions and Updates' page under the 'Environment' node:
Ensure that both checkboxes for automatic updates are un-checked as shown in the image above.