The received wisdom is that scammers often spoof the number you see on your phone so blocking a particular country might not work; some scammers also seem to show an international number but are based in the UK.
But, to answer your question, I don't think that it is possible to block a single country.
If you are using Plusnet Call Protect you can block all international numbers.. See the attached. You then have the further option to exempt numbers from the general block to allow calls that you are willing to receive. .
More generally, Call Protect seems to provide some protection but it is far from perfect.