I presume it's the short red line you want rid of? You're correct, it is a mini route which you probably accidentally created. It happens. If you have a paid account choosing the "selection tools" or similar should allow you to highlight and delete it. However as you're already a little frustrated if it's not causing an issue ignore it. In your image it shows "Start Trial" suggesting you're using the free version. You may not have the selection tools option available.
The auto-route change is frustrating and something I've never fully controlled but I do have a work around. I'm sure someone will be along to explain how to prevent this. It will be connected to a setting asking RWGPS to choose quiet roads, traffic free, possibly a shorter journey.
My solution is this. Rather than drag the blue dot I click on the road I want to use to create a way point. I then click on the next spot I want to reach. Usually RWGPS follows the road I want but occasionally will route me, for example, on to a nearby cycle path. Last night for instance I was planning a route where the road and canal ran parallel to each other. RWGPS routed me on to the towpath, I wanted to use the road.
The work around is to create a way point just beyond the spot where RWGPS chooses to use a different route. This will force RWGPS to go the way you wish.
I notice you're using Google maps for planning. Can I suggest you use RWGPS Cycle as the problem you describe seems to happen more often in Google maps. I usually toggle between the two map types but do most of the actual route planning in RWGPS Cycle. You can toggle between the two by choosing the drop down menu in top right corner of the map view. Toggling between the two will not affect the planning you have already done.