Hey (this might be a bit late, but this is for anyone else having this problem),
The solution is to clamp the Pitch of the Spring Arm constantly between values of -90 or above, and 90 or below, like so:
However, in most cases, if the player aggressively moves the mouse, they will be able to still invert it, which is why you want to set it to something like
75, so that there is a lot of dead zone where the player can’t push past. Hopefully there will be an option to disable Spring Arm inversion soon.