Something

class-map match-all bittorrent
match protocol bittorrent
!
policy-map bittorrent-policy
class bittorrent
drop
!
interface FastEthernet0
no ip address
ip address 192.168.1.1 255.255.255.0
ip access-group to-internet in
ip nat inside
service-policy input bittorrent-policy
speed 100
full-duplex