If Government decide to prohibit the acceptance of bitcoins or websites promoting the use, sale or transfer of bitcoins, this has a serious way of affecting the way bitcoin could grow, develop and advance. It requires collective efforts of many of all governments to stop it however.
I don't think they can't even do that. There's a lot of things they need to do before they can totally ban all bitcoin websites and etc. I've read a thread here of some people who still continue to earn bitcoin even if bitcoin is ban in there country and I don't know how they do that.