I used to work at a bank and once during our monthly meeting a couple of investigators from the DA's office came to talk to us about different scams; identity theft, etc. They told us to NEVER use a regular ball point pen to write a check as the ink could be "washed" and the check could be re-used. They said to always use a gel ink pen, that the gel ink wouldn't wash off. They demonstrated by putting some regular nail polish remover in a small bowl and dipping the check and using a cotton ball to wipe until all the ink was off. Some crooks are very meticulous and leave the signature intact; but that's not really necessary since banks clear clecks electronically and only spot check signatures. If there is such a problem, though, your bank will refund the monies lost after filing a police report.
I currently work at a bank and have been told by the folks in proof that gel pens will not be read by the scanning machines for the new check 21 policies.
Ask your bank about this.