That is FreeBSD's escape-to-debug keystroke (ctrl-alt-esc)
I'm not sure if there is a way to disable that keypress.
Though it does look like NMI would also go into the debugger, and that can be controlled via system tunables that you can disable under System > Advanced, on the Tunables tab.
machdep.panic_on_nmi: 1
machdep.kdb_on_nmi: 1