I assume your using the default layout.
Navigate to layouts/retro/styles/20-topmenu.otui
line 74 (may be different for you) but search for:
and add:
This will disable to top menu completely though, you can probably set the opacity back to 1 in client.lua or something when the player has logged in for example, if i had the time, i'd do it for you. But try yourself first!