Help - how to remove blu tac stains?

[freshin]

My son is moving out of his uni accommodation today and after removing his posters there are oily stains left on the walls. How can we remove them?

I can't help but I'm following with interest because my DD's bedroom is the same. An entire wall that she'd made a photo collage on so literally hundreds of greasy residue marks.

Honestly I think it’d be cheaper just to paint the room £20 tops for emulsion
scrub stains with cif first as there just grease