In the Heptabase desktop app, you can turn off the red underline for misspelled words directly in settings:
Open Settings → General.
Find Spell Check.
Switch it to Disabled.
To turn it back on, return to the same setting and switch it to Enabled. You do not need to type a misspelled word first.
Other spell-check options
On Windows, to show spelling corrections while editing a card on a whiteboard, select the entire misspelled word first, then right-click within the selection. With only the text cursor in the word, the card menu may appear instead.
You can still right-click an underlined misspelled word and choose Disable spell check. When spell check is disabled, right-click in an editable text area and choose Enable spell check.
When Spell Check is enabled, the same settings section also includes:
Spell-check languages: on macOS, use Open settings to choose languages in the system settings. On Windows and Linux, choose languages in the app’s language selector.
Custom words → Manage: add words that should be treated as correct, or remove words you no longer want in your dictionary. On macOS and Windows, dictionary changes also apply to other apps on the device.
These instructions apply to the desktop app. The web app does not have this desktop Spell Check settings section; spelling options there depend on your browser.