When I've had to clean spray paint from remote destinations, we've used chemical paint removers in spray bottles and scrub brushes.
Do not do this on a historic site without permission from whoever manages it. There is a very good chance you will cause at least slight damage while removing the spray paint, and if you are careless, you can make the situation worse than it was when you started. The manager is the one who needs to decide whether the risk is worth it.
Materials and setup
Solvent: First, find out what type of surface you will be working on. Then find a paint removal chemical that is safe to use on that surface. Paint remover is a solvent, so you want to be sure it will only dissolve the paint, not a wall.
Brushes: Once you've got the solvent, get a few brushes and spray bottles. Make sure the bristles won't be dissolved by the solvent. Depending on the solvent, you'll probably need natural bristles. Metal might work, but it will cause much more wear on the historic site's structure. Brushes of different sizes and shapes are convenient if you've got an irregular surface, like natural rock.
Spray bottles and water: Fill half your spray bottles with solvent, and half with water. Then take extra water with you because you need enough to really rinse the solvent off the site when you are done. Figure on 2-3 times as much water as solvent, maybe more.
On site removal
Now it's time to take the materials to the site. Test the chemical on a tiny, hidden patch first. Spray the spot, let it sit for 15-20 minutes (or longer if the chemical's instructions say to), then wash it off with water. If the chemical hasn't damaged or discolored anything, you can use it (cleaning dirt is OK). Make sure you read the label of the chemical.
Now you are ready to start removing graffiti.
Do No Harm: First (and MOST important), think about the spot where you are working. What is the graffiti covering? If it is near or on top of an older painting, leave it alone. Same goes for if it is near writing, fabric, or anything else other than the material you've tested. You will do too much damage removing the graffiti. An expert can clean a single layer of paint off another painting, but you don't have the tools or the knowledge to do so. Do not even try. If you've got any question about whether you can safely remove the paint, leave it. Even if it takes another 50 years, someone with the proper skills can deal with it later, but if you cause extra damage, you make their job much harder, or impossible.
Scrub: If the paint is on a plain wall or surface, you can start removing it. Follow the instructions on the solvent. It will probably say to apply it, then leave it for about 10 minutes before removing it. The longer it sits, the better it dissolves the paint, and the less work you need to do. Scrub until the paint is gone. Rinse with the water as you go. Remove as much as is practical. There may still be flecks left, but they will be relatively faint.
Good luck and happy depainting!