Search for the OP username and see if they have posted in India subreddit, if they have block them.
When you open a comment thread on Reddit, a search takes place from the client looking up the OP username to see if they have posted "india".
https://www.reddit.com/search/?q=author:${username}+india
If there are any results from this search, the user is blocked and a popup lets you know you are posting in an Indian thread.