New .in domain being used for spam

[MARKA]

I have been noticing over the past few days that my site is being inundated by spammers using the .in TLD.

The email addresses are like this:
@alpha.centauri.minespace.in
@etno.mineweb.in
@gliese.minespace.in
@eclair.minemail.in

As you can see, they all use .in as the domain. But, since they are changing the second level and sub-level domains, I don't know how to ban these email addresses. I don't want to enable aggressive email banning because that would snag too many innocent people.

Is there a way to disallow anyone using the .in top level domain?

My site is for U.S. military, so there won't be any legit registrations from India.

Thank you in advance for your help!