Boom Boom can source some brilliant bats, they're not all made by Ihsan I think are a few other sources all of whom are reputable batmakers .
Nets is always a grey area in a player contract, it doesn't say explicitly they have to use their sponsors gear in nets. They are however paid to endorse/use their said manufacturers equipment and thus it is expected that they use the correct equipment at all times, particularly when there is a possibility of being photographed. Player do like to try each others kit out though, hence why you'll occasionally see players swinging other brands of bats about.
Contracts are usually signed for a year minimum. There is however an upcoming trend of players using bats for particular tournaments, namely the IPL and Champions League.
Termination cannot just happen because you want to leave, remember there is a contract in place. There are of course clauses under which you can terminate, which can range from gross misconduct (failure of drugs test, matchfixing etc) to the brand going into liquidation. Another popular termination clause I've seen is non-supply of kit.
It's interesting Afridi, Alam and Amir have all stopped using Boom Boom in nets. And of course the reasons are unbeknown to us. But as you can see from my above post there could be any number of reasons for it and come the next game they may be using Boom Boom.