I knew I wouldn't be able to find the method until I asked about it.
The answer, at least on Android, is that the upper right menu icon is context specific, and doesn't always display the "settings" option.
1. Using the app nav bar, go to your list of forums.
2. Select the menu icon (three stacked dots in upper right corner).
3. Chose settings
4. About half way down there's a signature setting, which can be customized or disabled.