How to stop unauthorized users?

I have one wireless connection here in the camp but its only for managers.i have put cisco aironet 1300 AP to provide wireless connection and as this Ap doesn’t provide the dhcp service so i have put dhcp server to assign ip add.i have put password for the connection for the security but somehow some unauthorized users are getting the password and connecting.i can see them on the router and dhcp server but cannot block them.kindly suggest me what to do.
Thanx