Mix a weak solution of hydrochloric acid with water in a glass container. The acid will easily eat into plastic and it may react with a metallic container. Use six part water for every part of acid and stir with a glass rod. Add acid to water and not water to acid. Before you start handling the acid, wear rubber gloves and goggles to protect your skin and eyes.
Test to see that the solvent is not too strong or too weak. To do this, replicate the ink stain on a scrap of paper of similar quality. Place a few drops of the acid solution on the ink stain with the glass rod and observe its action on the paper and ink. If the acid destroys the paper, dilute the solution further by adding another part of water. Keep testing until the acid solution dissolves the ink without damaging the paper.
Place drops of the acid solution over the ink to be removed. Add a little and observe the effect. If the ink starts dissolving, place more drops of the acid until all the ink is gone.
Dab the spots where the acid solution was dropped with water. Rinse out the acid to stop its actions. Then let the paper air-dry.