Same thing just happened to me!
If you have a computer (PC or Mac), you can use the instructions here to bulk delete Garmin Connect activities by device.
If you need to do the same for Strava, their support team suggests you get in touch with them.
support.strava.com/.../216921897-Deleting-activities-in-bulk
Unfortunately, we don't have a bulk select tool to delete multiple activities at once. The most accessible way to delete activities one by one is from My Activities or your Training Log under the Training tab. If someone else's Garmin history was uploaded to your Strava account by accident, please get in touch with our support team directly.