I lost some customers to someone doing this a few months ago ... I was new, so oculdn't really expect instant loyalty.
So, I prepared a letter saying that I didn't want to alarm people, but a couple of rogue cleaners had been canvassing in the area, quoting extremely low prices in an effort to gain access to houses they had no intention of cleaning.
I worded the letter to strongly imply that these people were interested only in criminal gain. And although some might find that a bit sneaky ... it worked a treat. Not one person who received the letter dropped-out, and I had several comments from people thanking me for alerting them to the problem.
What;s more the couple that I did lose, no longer have a window cleaner, and yours truly certainly won't be taking them back on.