How do you stop an activity when using Training > Set a Target on the 255?

If only Garmin would add the option to stop the activity automatically when the target distance has been reached. Such problems would disappear.

I doubt it, as most users would not select that option, as you would probably miss parts of the workout in the log. 

How would you miss parts of the workout - if you've specifically configured a 5K workout (for example) - then you almost certainly want the activity to stop at 5K. As it stands you always end up with something like 5.01K being recorded - with a few extra seconds tagged onto the end of the run. It's very frustrating. I know a few people who run with Garmin watches and the current functionality is universally disliked - all of them would select the option that I'm suggesting.

If you compete in a 5K, or want to run a 5K strava segment, you want some extra log at the start and end of the run to make sure you captured the start line and goal line. Besides, you want to let the watch soak the GPS properly before you start it, so that will typically include some of the warmup since you press start before you cross the start line. If you are in a large event, you will perhaps line up a few hundre meters before the start line, and record the running to start. 
If you are on a running track, the GPS could be slightly off compared to the actual laps on the track. That is why everybody I know stop the watch manually after the distance is finished. If you want to trim the log to exactly 5K, it is easy to do after you get home. If you missed the start or finish, there is nothing you can do. 

I understand why you want it, and I would not mind having that option. But I think for most people it would not be used, and the bug described in this thread would still be an issue.

Fair point.

Maybe instead of using Training —> Set a Target, create a workout for the intended distance with a warm up before and a cool down after. 

Interestingly this does allow you to stop the activity once the workout complete message pops up. I wonder if this actually is a bug on the 255? I tried using Training > Set a target on my 245 and sure enough the activity can be stopped from the summary screen. Anyone know what the official channel is to submit a bug report to Garmin?

tdembows said:

The stop button should always stop the activity, no?

How you can see - no. But you can press "Back" to disable info before it desapears and then "stop/pause" to stop your training.