It appears that check fraud has become popular again. Con artists are 'washing' old checks to remove the ink using common household chemicals. After they remove the ink from the checks, they're cashing them for any value they choose.
To fight back, you need to use a pen, that has an ink that is that can't be washed off. For example, the Uni-Ball 207 uses an that is almost impervious to many solvents . This pen can be purchased for about about $2 a piece.