For starters, I never go back to any supplier which requires this if I have any choice. Sometimes there is no choice, for example, it's my understanding that all car rental companies in Israel require this.

I also try to write "Signed in blank" and that has worked for me. I have also immediately called the credit card company and reported it to them although many of them will just say "You are stuck" if you try that. Still, calling them, and having a record of who you warned about this, while it might not help, certainly couldn't hurt.

Car rental companies -- even majors like Hertz and Avis in some countries -- seem to be the worst offenders, although in Israel, I was asked for the same thing when I wanted to rent a cellphone from someone at the airport. In that case, I just decided I could live without the cellphone when there.