A bit complex, as the event type selection happens on the form, so it would need to dynamically disable it. But a good idea.

I've documented the issue in the tutorial.
I have made it so on editing a system event WYSIWYG is disabled.

This should deal with the bulk of it.

Alternate solution (on top of my earlier tweaks): I've just hidden this event type for new events, unless a config option is flipped. Nobody uses it anyway, it's very much for power users.