My tool of the month Transform Rule followed by a Firewall Rule would be easier.
In general, this is a bad idea and a poor user experience. As @sdayman said, an efficient website with a good caching policy should make the number and location of users irrelevant.
I can think of lots of reasons why blocking by country/region is bad, some of which depend on what your business is. The easiest one is that geo-IP data is not precise, and you might be fire-walling people by mistake. The less easy one is that if I’m travelling to
<city> next week and need to find a
<dentist, hardware store, toy shop, etc> when I get there, I’ll never visit those businesses who just had a big error message instead of a website. But there are plenty of other reasons.