You need to configure people picker to search only specific OU in your AD directory. Have a look at this question
As Marek said, you can limit People Picker to a certain OU in Active Directory.
If a Web application is using Windows authentication and the site user directory path is not set, the People Picker control searches the entire Active Directory to resolve users' names or find users, instead of searching only users within a particular organizational unit (OU).
To restrict People Picker to a certain OU in Active Directory, type the following command:
stsadm -o setsiteuseraccountdirectorypath -path <Valid OU name> –url <Web application URL>