I'll second Mike4 (although lighter-fuel will work it stands a risk of dissolving the surface).
Another (less aggressive) option is FRESH white bread 'middle' rolled into a 'putty' and then used as an eraser (you could use it as a rubber, but I wouldn't guarantee it
). 'Dough' erasers are recommended for cleaning wallpaper. You could also use a soft pencil eraser (the sort that draughtsmen use). NOT a hard red or green 'ink' eraser!
I'm assuming that the marks are on conventional wallpaper (not silk or other exotic covering). If it's a painted wall, (EDIT I see it IS a painted wall end of Edit) then sugar soap (or even a weak solution of washing-up liquid 'painted' on using a soft paintbrush (but be ready to wipe away any runs). A soft nail-brush dipped in dilute washing-up-liquid would also work.
One problem with 'blue-tak' is that when dried and hardened it can strip-off the surface-layer of wallpaper, leaving white blotches. There's only one solution there - new wallpaper.
Try an inconspicuous area before you ruin your decor even further!
PS Blue-tak can be rejuvenated by mixing it in a ball with a little neat washing-up liquid 'injected', then running the slimey mess under a cold tap (this removes the dirt). Afterwards 'roll it out' (like pastry) using a rolling-pin or smooth cylinder until it reaches the required sheet thickness, then pack in grease-proof or waxed paper (like the original stuff comes in). It can be re-used year after year (we use it to hold up the Christmas Cards on glass and gloss-painted surfaces). You can repeat the 'cleansing' process several times, provided you rinse in-between to remove the dirt and sludge.