MAC Address filtering sounds like the best solution to your problem.
If your PLDT modem doesnt support that, then buy a router that does (like a Linksys), disable the Wifi on the PLDT modem, connect the Linksys to the modem and connect via Wifi to the Linksys.
Adding all the MAC addresses can be a pain in the rear, but can be worth it.
Also, make sure you are using WPA2 encryption on your access point. WEP and WPA can be cracked.