If it really really bothers you, paint the walls with clear polyurathane. Comes in serveral finishes, and I guess "flat" would be be the best, unless you prefer "high gloss" walls
In my photography persuits many of my darkroom building buddies built their own processing sinks out of 2x6 and plywood. The inside of the sink was painted with several coats of poly and they held water really well. So some poly well applied should waterproof your walls. Test a small area to make sure applying the poly doesn't smear the ink in the wallpaper. It shouldn't but make sure.
Get plenty of ventilation in there while you do it, let it dry for a week before you get it wet. I seem to do better with a brush than spray on.