is it possible for a client to avoid p2p packets dropping via http tunneling?
Yes, I belive so. If the traffic is encapsulated, or encrypted, I don’t think that it can be identified as p2p.
is it possible to detect if a client is using a http tunnel? is it possible to block this kind of stuff?
you have to know what kind of tunnel he/she uses, if pptp then if you drop GRE protocol - tunnel cannot be created, if l2pt than this is other story.
i would suggest you to limit traffic for that customer - and if he/she complaints, that has no speed, than as an answer would be - you have XXXKB/s limit set as you used p2p software - turn it of and everything will work for you. 
at least it works for me. as i limit bandwidth for user not for what he/she does. so if he chooses to use all available bandwidth for p2p it is his/hers problem not mine
openvpn is a good way to avoid this. plus it can’t be detected as a tunnel