I did a quick read up on mobile internet in the Philippines. Is the 3G connection that bad at the shop that you need to use WiFi? I suspect that this is reason the question came up in the first place, but I thought I make sure regardless. In most of Europe as well as north America most people would probably assume that customers have a 3G/4G data plan if they pay with bitcoin and dont bother at all.
More towards the actual topic, I would not try to block all but bitcoin on the port layer, but go to the application layer (7). If you have a router that can be run with open firmware solutions like tomato or OpenWRT it should be possible to create a bitcoin only WiFi. This might get tricky to setup though as I doubt there are ready to use filters like there are for other p2p traffic like torrents.
Maybe you can find a "good enough" solution by blocking HTTP(S) and mail as well as other commonly used services that generate a lot of traffic without actually having an airtight solution.
Shorena, 3G connection is readily available in the Philippines. The problem is the cost (hence the offer of free WiFi in the shop). Minimum connection fee to 3G can cost you PHP10 for 30 minutes minimum when the average salary is about PHP300 for a 10 hour day. Your internet connection using a smartphone can cost you almost as much per minute as you can earn per minute!
Thats expensive yeah.
The way I would approach this is as I wrote above (now with more links): get highly customizable firmware for the WiFi router and do some heavy research on layer7 filtering[1]. You would need to write your own filters though as I can not find any for bitcoin, besides this[2]. I am unsure whether filtering by the "magic number" for mainnet would work for apps. The easier route would be to just get as many layer7 paterns as you can[3] and just block everything else. Its certainly a longer project as the filters might need updates over time as customers run into problems with certain wallet apps.