Ah! That sounds really frustrating, sorry about that. It looks like a bug is causing the fonts to not delete. Please try the following steps:
1. Open StimuWrite. Go to Advanced Settings and click the red "Delete All" button. (This will delete all themes and recovery files and supposed to delete addons)
2. In your computer's file explorer, go to this folder:
Windows %APPDATA%\Godot\app_userdata\StimuWrite\
macOS: ~/Library/Application Support/Godot/app_userdata/StimuWrite/
Linux:
~/.local/share/godot/app_userdata/StimuWrite/
3. Manually delete the addons folder
4. Open StimuWrite. Download and install the add-on again. Make sure to use the original file name.
Please let me know if that fixes the issue. I'm not sure what caused the bug, sorry about that.