How do I disable changing of birthday?

[kvnband]

I would like to let the user either:

A: Set their birthday once, and then never be able to change it again (unless they contact me) or
B: Have admin functionality to enable and disable changing of birthdays.

My idea is to tell users that something special will happen if they set their birthdays (I'm trying to get as much user info as possible while at same time making the site better). Users will have maybe 2 weeks to set bday in profile. After that time, I disable the changing of birthdays. I then announce that on a user's birthday, they can make one request for something to be done for the site. If it is reasonable and will help the site, the request will be granted (our site is a very close knit group of individuals all striving to make the site better). If I don't disable the changing of birthdays, then users can change their birthday to tomorrow so that they can get their one request. I don't want this to happen.

Any ideas? I guess I could just remove the birthday field from the edit profile page. That would definitely do it. Would that be the best way?
Thanks,
Kevin