And keeps receiving more money from everywhere due to all those pesky tourists, ensuring that a foreign currency flow would never completely halt.
Well, that would be when they bring the drachma in (assuming no Eurozone support). In the intermediary stage, given they'd have to use Euro coins and notes as drachma stand-ins, things would be more difficult (if they decided to print "Greek Euros" to make sure circulation remained high). They wouldn't be allowed to trade with other countries while this happened as otherwise it'd just invite arbitrage or a wholesale currency collapse in Greece. There's an article on it here:
http://www.bbc.co.uk/news/magazine-18279522 (well, the article's a bit vague as it was written some time ago, but you should get the drift). Personally, I think the six month time-scale is pretty alarmist. Greece has been prepping for a drachma return. I don't think the interim period would be particularly long; but during that interim period exports/imports (including tourism) would effectively stop thanks to said capital controls.
The alternative is they just continue to use the Euro at the current rate until they can bring the drachma in but I'm not sure they have enough Euros to do that.
This is all a bit hypothetical though because honestly, in the event of a currency switch, it's in the interests of the Eurozone to allow Greece to transition smoothly and I imagine the Eurozone would just give them the Euros they needed until Greece had prepared the drachmas - after all, that's less Euros than they're currently set to "give" them in the form of a haircut. The above is just assuming that, as tapantaola said, the Eurozone decided to just fuck Greece for the sake of fucking Greece, which I think is basically fanciful nonsense. The Eurozone fucks countries for profit, not fun.