If that doesn't work, when I used to take an oil painting class, we used turpentine to clean the paint brushes, so perhaps that may work? I don't know much about interior paint, but if you do give turp a go just be careful because it is quite unhealthy to breathe in, and you'd have to be careful not to get any on either you or the actual wall where you want the paint to stay

Make sure you run a blade across the top of the tiles first to create a break line - you don't want it to peel up the wall!