How to kill htop if I can't use F10?

TL;DR The standard way to quit htop is F10 or q.

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Therefore if you can't use F10, use q (lowercase).

You always can have the interupt signal keys Ctrl+c.

Basically Ctrl+c sends the SIGINT (interrupt) signal, ; by default, this causes the process to terminate.

Just like top, htop can be quit by pressing Q.

SIGINT

The SIGINT signal is sent to a process by its controlling terminal when a user wishes to interrupt the process. This is typically initiated by pressing Control-C, but on some systems, the "delete" character or "break" key can be used

If you are using gnome-terminal, yes, you can disable that the F10 key opens the menu:

Open gnome-terminal's menu with F10 or with the mouse, select Edit > Keyboard Shortcuts, then uncheck "Enable the menu shortcut key (F10 by default)".