I have the same problem. To remove that strange max distance you can go to Garmin Connect and delete your golf club Driver and then you can add again the same golf club. Most probably statistic data of removed club will be lost.
First, congrats on that 38,963 yard drive. Well done.
To get rid of the drive, it's very easy but requires several steps. Go to the scorecard within Garmin Connect. Once you have the scorecard, you have to find the hole that had that drive by clicking on each hole until you see the drive (time consuming I know). When you click on the hole, you will see all shots that were recorded. Then click on "Edit/Add Shots". Once you see the drive you want to delete, click on the three dots to the right of the club and select "Delete". That's it.
It would be nice if Garmin had an option where you could put in the maximum yardage that would be recorded (i.e. anything more than 350 is not recorded).