I don't think is wrong to remove inflammatory text from a forum. The issue comes from editing the text out, because then you are in the grey area of what parts of the speech are inflamatory and what parts are not.
Update the policy to say that any inflammatory or provocative post that don't follow the previously stated rules (the ones you posted) will be deleted/removed from the forum, and the poster will be invited to post again abiding the rules.
I asume you value those contributors, and the issue is only that they get heated up when is about politics.
Have a private talk with them, explain them that their behavior only makes them look bad. And it also affects the forum and their country image.
When I was little, I was told that I was my country's ambassador. My behavior reflects on what other people think of my country. Someone that attacks others the way you describe they are, only accomplish to damage their own country image.
Comparing countries is like comparing apples and oranges, there is always going to be someone that like one better over the other, so it is pointless. You can argument that a specific policy is better in one country than the other, but that's about it.
Finally if they can not behave properly in a politcal forum, ban them temporarily. Make it part of the general community rules. They can not complain about censorship since they are being disrespectful and are not following the interaction rules the site has. Also this is pretty standard for communities.
If they continue with the bad behavior after several bans, ban them permanetly or for a significantly longer time, or if they leave the community altogheter, it is better for the community, people that can not follow basic rules of courtesy are more damageful than good.