This question is old and still seems not to have "clean" solution like disable button in menu but I want to share my workaround on Edge 540.
Steps I did:
- start a course
- stop a course
- go to Virtual Partner option under Controls menu (it looks that without first step this option is not present in the menu, at least in my case)
- change speed of VP to 0 km/h or enormous big number or whatever you want just not to disturb you while riding
- re-sync to User
If you use several profiles, and want to have the same workaround active on them, you have to switch to each profile and repeat these steps on each profile you use.
This procedure seems to give permanent result.
You can edit VP setting while course is started but then you can change percentage only by which VP will be faster or slower and each time when you start again the same or different course you will see default value of 100%.