There are a few situations that can trigger another person's Garmin data uploading to your Strava account. One is if you borrow a friend's Garmin or acquire a used Garmin, the device's activity history can be synced to your Strava account when you use the Garmin Connect app to upload wirelessly. In this event, proceed with submitting a support ticket for assistance in bulk deleting the history from your Strava account. It's also possible that Garmin Connect authorizes to the wrong Strava account. If you find activity on your Strava account that is not your own, navigate to your settings, disconnect Garmin and delete the activity. If desired, reconnect to your Garmin Connect account.
If multiple users upload with Garmin Express on the same computer
Make sure to associate each device with the correct Garmin Connect account to avoid uploading data to the wrong account. First, open Garmin Express and select a device from the drop-down on the upper left. Click "Device" on the upper right and scroll to the bottom of the page and look for "Garmin Connect Account". If you need to change the account for a device, use the "Change Account" button.