Is there a way to prevent the hot-end straying over non-printed areas?

[cnd]

I'm printing a tall thin part - a turbine blade. It is fat at the base, skinny at the tip, and twisted along the length.

The hot-end is, for no sensible reason I can imagine, moving over the non-printed area at times, and when it returns, it is knocking the top of the print.

Is there a way to force it never to go outside the print area, so that can never happen?

The part (it is two blades, joined at the trailing edge)