Problem In Password Changing

Question

I Have A visual web-part of changing a password of team site(it automatically changes the password in Active directory but it is taking 3 min time to update the Active Directory in   the mean time user can able to log in into both the password new one and the old one )

1)so how to reduce the time interval

2)else is their any way to update password in active-directory  

 3) Or how can i handle these time period(what i mean to say is shall  i lock the account up to new password is updated..? is it possible..?)