Hi ,
I suppose you can do this by plenty of ways ...
1) We can use the option of access list to deny any one joining the netwrok.
2) We can implement WEP or WPA , so that no one can get authoraization with out know the passwords .
The better way to use acess list and allow only those clients whom you want to grant acess.
This will even obivate the amount of processing power required for WPA or WEP
Regards,
Bramnha Prakash Tiwari