Question

you visit a place of historical importance with your friends and come across people who subscribe their names and messages on the walls of the monuments and you feel this act not only showed people's indifference for their culture but also reduced the beauty of these monuments

Answer 1

I will ask them to stop doing that and explain them about the mistakes they have done towards the surroundings....

Answer 2

In such case, first I gently explain to such persons that by writing on the monuments, you not just only spoil the natural beauty of the area but also you destroy the evidence of our culture and religions.
And then I take promise from all of them that they should never do this again.
If they still not follow it then I complained them to the security council.