Login to JustGiving and then go to ‘Account & preferences’ via the menu next to your profile picture in the top right of the website. And then go to ‘Apps' on the left hand menu and you’ll see a ‘Disconnect Strava’ link.
Or go directly to the ‘Apps’ page, after logging in: www.justgiving.com/user-account/apps
You can also disconnect JustGiving from within your Strava account: https://www.strava.com/settings/apps
Can I disconnect/pause just one of my Fundraising Pages from Strava?
You can stop activity from showing on a specific page by selecting the ‘Manage Strava Settings’ button on your Fundraising Page (you must first be logged in to JustGiving) and either unselecting all the Sports Types, or by selecting the ‘Stop sharing from Strava’ button at the bottom. You can show fitness activities again, by simply selecting at least one Sports Type.