As per instructions in the FM Online manual "Customizing Frame Products", here are the steps to disable the popup Format > Fonts display menu.
1. assuming that you do not already have a customui.cfg file: create a new text file named "customui.cfg".
2. add the following line to disable the fonts flyout:
<Remove !ShowFontChoices <Menu FontMenu>>
3. if you want FM to read this file automatically when FM starts, place it in the \fminit\configui subdirectory. Note that this is the directory as specified in the maker.ini line "ConfigCustomUIFile=fminit\configui\customui.cfg"
If you want to load the customui file manually, place the file outside of the "configui" subdirectory; when FM opens use View > Menus > Modify to read the file.
4. if you already have a customui.cfg file, the "Remove" line above can probably be added to it, but for safety be sure to make a backup of the original before modifying it.