Some governments have already tried that but miserably failed
The Chinese Central bank tried something to that tune, even twice but Bitcoin only rallied further and higher. "Freezing" exchanges would likely mean stopping Internet at large (to be effective), but that would be prohibitively expensive for obvious reasons. Otherwise, how are you going to stop something which might not even have physical presence? There are no offices to get raided, there are no bank accounts to get blocked, so how are you going to halt the exchanges? Some countries block access to exchange sites, but there are free VPN's and proxies to easily render all these efforts futile and void