You can talk to ppl how it makes impression of India on tourists You can talk to ur sculpture teachers n parents to provide some ideas You can discuss it wid ur frnds You can write letter to govt officer discussing the issue You can take out rallies or campaigns to provide awareness
All we can do to stop these things is to make them understand  that pasting posters,writings etc... will hide the beauty of our monuments. It will create a bad impression on the tourists coming in our country. And it is our duty that we should not spoil these precious monuments.
instead of monuments you can write bridges,traffic signals etc....