It's not specifically illegal, so you can't force them to not do this. IF your wife suffers identity theft, etc. due to the fact that her social security number is on the check (you'd have to be able to provide some evidence linking the SSN on the check to the theft: e.g. was done by your babysitter or cleaning service (if you have one), who saw the check laying around, or by someone in the loan provider's mail room, etc.), you could sue for that loss,  but again, you can't affirmatively or proactively prevent this.