disable the windows key in parallels 14
Open Parallels Desktop and go to Preferences > Shortcuts > Windows 10 (or whatever your virtual machine is called, in the Virtual Machines section of the sidebar). Add a new keyboard shortcut and map the cmd key to ctrl, or something other than win.

This will definitively stop the Start menu from popping open every time you hit the cmd key individually, but I'm not sure how it affects other shortcuts that make use of the Windows key.