You need to edit it in an image editor such as Photoshop.
Right click the page with the TouchUp Object Tool and choose Edit Image. This will open it your image editor, where you can make the changes.
Thanks for that. Had just hoped there would be something that did a 'clean' type function in Acrobat.
Actually, you can use Acrobat for that. OCR the pdf with clearscan. Then click on "edit text and images." It will show you boxes around text, images and even small marks on the edges or anywhere else. Just click on the box and delete it. If the "dirt" marks are close to the text sometimes it is connected to it. In that case right click on the box you want to remove and select "edit using ->microsoft paint." In the paint program you can delete the unwanted markings around the letters. I hope it helps.