Unwanted (not mine, someone else's) courses on display

Are you on Garmin Connect? Make sure you click on the "Created by You" tab

Thanks for the quick reply. I haven't got anything in the Connect app under 'created by you' as the course I created is a imported gpx file that I copied directly to my 830 memory. 

Could you post a screenshot or pic of what you're seeing?

Here's a screenshot. The large white pinkish trail is my gpx import but what are the blue, green, orange, red and thin pink lines???

The green are segments. Blue is typically your current activity though it might be a river depending on the map you are using.. The thin pink might be roads/trails as it looks like you are in night mode.

Thanks for the colour info, that helps a lot but Im still left with the question of what are the smaller pinkish trails on the map? They may well be trails but I know the area well and there are a lot more trails there that aren't highlighted in pink! Why can't I turn them off just like all the other trails on the map that aren't highlighted?

Have you enabled „popularity map“ in the map settings?

Are you using an MTB profile? The pink lines are probably single trails from another map layer, very probably trailforks. You can enable/disable which map overlays you want to have active for each configured bike profile.