You can't personally block people from being able to see your Twitter accounts based on their country. Twitter itself, however, has the ability to withhold content from appearing in certain countries. Here's why.
Twitter has to make sure that local laws are respected in terms of content. Something that is deemed acceptable to view in one country may not be somewhere else.
In these instances, Twitter may block content from being viewable in certain locations. Official organisations may also request that Twitter refrain from allowing its residents to view certain types of material.
How can you stop people from seeing your account?
Although you can't block specific countries' users from being able to see your Twitter content, you can individually block users. This is obviously something that will be pretty poor in its effectiveness as you're not going to be able to block every user of a particular country individually.
On the other hand, you can block individual users from being able to view your content or interact with you, and this is a pretty convenient tool.
You could also take the opposite approach and make your profile private. This way, only people you authorise will be able to view your profile.