Of course they can control it but only to a certain extent: they can decide what companies can or can't do legally, they can decide which websites (exchanges, pages of projects, whitepapers etc) are visible on the web and which websites stay out of reach for a person of that nation, they can make sure you pay taxes on your assets. BUT what they can't do is find your secret hidden wallets. I also don't think they can stop someone who really wants to buy any altcoins any time from doing so if the person really knows how to use computers.