Hi Sjaak,
Indeed it will became a little be more complicated, but there is solution (there is always a solution )
First, you have to create two zones. One zone with the countries which can't buy and the other one.
Then, you have to create two manual shipping methods (I know you don't send goods but don't worry, it's the only option).
Make these two methods free and add a restriction on them -> method 1 to the first zone you created and method 2 to the second zone.
Once this is done, enable the option "Use price per product".
You can now go to your product configuration page (product you want restriction on) and you'll see a new block on the bottom right (shipping price).
So just check the "block" checkbox for the shipping method assigned to forbidden country. People from this country won't be able to buy it from now.
This should work so far. Last step if you want something really clean, add some CSS to your css file to hide the line with the shipping method so people won't see it when they pay.
I hope it will solve your problem