What can be used to wash off the slogans on the banners?

Use cleaning fluid. A bucket of cleaning fluid costs about 200 yuan. It is only for inkjet printers. It is not sold in general places. You can go to an advertising company that has inkjet printers and buy some that usually won’t ask you for money. Don't buy too much, the ink tank lasts only a long time. Dip it in with tissue paper and wipe it. Be careful because it is corrosive, so don’t smear the color.