Not all Bananians are the same. I know that there are a lot of Bananians which are quite annoying due to their lack of English and general education paired with their desire to articulate themself (might be a cultural thing), but to generalize this to all of them is quite racist (There! The R-Word! I said it!). Maybe you have some Bananians in your community which are constructive members and you don't even realize that they are from Bananastan because they fit in so well?
Also, keep in mind that those masses of users who spam your site with their Bananastan IP addresses might in fact be fewer actual people than you think. Bananistan has only one major ISP, and that IP uses dynamic IP addresses, so the same user can appear under lots of different IP addresses. Maybe you are in fact only dealing with one annoying user with many identities.
However, in the end it is your decision. It's your community, you make the rules.