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If a human, using his own spacecraft (built with private funding no government money) lands on Mars, could he declare to be his property and prevent future humans to land on Mars? Would we be obligated to respect this restriction?
What if instead of a human is an alien who lives on Mars, if he declares Mars as his property, would we respect that and never travel to Mars? Or the governments would ignore him and travel anyway.
Fundamentally, ownership or legal possession is a societal construct. I only own my house/car/computer/patent/whatever because I, everyone else, and especially the government believe that I do. If someone steals your stuff, you expect your tax dollars to help you get it back/prevent further theft. There is no magical galactic law of property which defines ownership, it's just something we agree on.
Now, what happens when someone disagrees on ownership? In places with law, you'd go to court where the different claimants argue their cases and the government (or someone else legally binding) adjudicates. When a decision is made, for example person A owns the property XYZ, then the government promises to protect person A's property from people who would steal it (with police, military, etc).
In places without law, eg the archetypal Wild West, you can impose your own version of law through the application of force: for example, the cowboy's revolver. The cowboy gets the other people to agree that his herd of cattle is his, not because of some law, but because he will shoot anyone who says otherwise. A gang of bandits, who disagree, can claim ownership of the cattle by shooting the cowboy.
Currently, there is no enforcement of space law. No country has space marines, space police, or star fighters to back or defend claims. Yes, there are some (rather flimsy) agreements between Earth-nations not to do anything untoward in space (eg. the OST) but this primitive space-law has only been upheld because no country has had any reason to break it. If suddenly proverbial oil were found on the Moon and it were profitable to get it, the OST goes right out the window without a second glance.
Thus, to answer your question:
If you land on Mars or the Moon or wherever, establish a claim, and shoot anyone with your six-shooter if they disagree with your claim, it is effectively yours until someone with a bigger gun comes and takes it from you.
Under international law, no. Google it. Lots of companies claim to be able to sell real estate in space, these are scams.
However, your Martian would be in an excellent position to disrupt any other landing attempts and governments might take their threats seriously. NASA can't afford to put anti-tamper devices on their rovers. :)
They can't
As far as I know, all space is considered by law and in practicality to be neutral. This is practical, as it would be impossible to use satellites to fly over only friendly territory, but also prevents a lot of hassel with claiming things in space. The moon isn't American territory.
Being the first to set foot there also isn't a great basis to claim something. Discovery is more important.
But you might still ve able to claim something. If you build a relatively permanent settlement there, you'll likely be able to claim something, as it and some surrounding area might not be considered space anymore. In addition laws will change to accommodate for these changing times. You can't keep saying everything in space is neutral, but at a certain time that's not feasible anymore.
International law forbids, but international law forbids a lot of things. One of the key things that trumps international law is one of the oldest laws in the book, sovereignty. The other thing that trumps international law is power.
Once upon a time we recognised the Republic of China, now we recognise the People's Republic of China, and Taiwan, but Taiwan isn't a fully recognised country due to the influence of China, which claims the territory as its own. Power and money hold sway, regardless of what the law might say.
There's the claim, and then there's the ability to enforce that claim. While an individual landing on Mars could theoretically make that claim in spite of international law, what really matters is their ability to enforce that claim in the face of international law and other opposition.
He could make the claim, but could he make it stick?
Under law, it is strictly prohibited. You may find a lot of scam websites or companies who sell land on different planets, including Mars. If it was allowed, then the first people who land on Moon would claim that "Moon is ours, from today nobody will travel here". And also what he would do with that? He will be defeated in a few moments if he is alone and does not have weapons. Space is neutral and not in claim of any organization, people or company. And you have said about Aliens, we don't know. Humans only fall under law, not aliens. Aliens if enoughly weaponized, can defend us from accessing their planet.
