Rather than worry about questionaires for candidates who don't take up your offer, concentrate your efforts on fixing the recruitment and interview process to raise the calibre of applicants. Also make sure you are offering a workplace that people WANT to work in, i.e, offering advancement, job fulfilment, decent salary, etc etc. Your interview process should be able to weed out only those interested in money and nothing else.
Secondly, no-one is going to bother filling in a questionaire. Would you if someone sent one to you? You're too busy starting your new job with another employer to worry about this nonsense.
If you are worried about why people don't want to join your organisation, then pick up the phone and TALK to them. Develop four (4) questions to ask, and don't waste either your or the other person's time with this. A short sharp conversation and end the call. Many people won't want to discuss it with you, so be prepared for that. Also be prepared to be told things I suspect YOU WON'T WANT TO HEAR.