If a unit is stunned can it still be moved by a spell or ability?

Yes, a stunned unit can still be moved by a spell or ability.

According to Rule 423.1, the Stun status only restricts a unit in two specific ways:

1. It prevents the unit from contributing its might to combat damage (Rule 423.1.b).
2. It prevents the unit from being stunned again (Rule 423.1.a.1).

Stun does not restrict a unit’s movement, nor does it prevent the unit from being targeted by effects that move it. As noted in RiftJudge FAQ #10103, if a card specifically includes a "cannot move" restriction (like the ability of Vex, Apathetic), that is an additional restriction imposed by the card's text, not by the Stunned status itself. Unless a card specifically says it cannot be moved, a stunned unit remains a legal target for movement effects.