To Mr. UtimaOnline.
Even Planet Earth's great scientists such as the genius of his scientific field - the late Stephen Hawkings, believed that we are not alone, & just by mathematical calculations, there are PROBABLY other forms of civilizations in the Universe, which even mainstream religion such as Christianity & Islam acknowledges. However, to achieve INTERSTELLAR TRAVEL, such beings would have to be at least TYPE 2 Civilization - far higher than the self serving Humans let alone NATION/SOCIETY common on our planet. as per the renown scientist Michio Kaku. Such civilizations are beyond greed, corruption, etc, & their purpose is to only expand knowledge of the greater Universe they live & survive in, having SUFFERED much & rose beyond such mortal trappings into such.
While there had been many physical manifestations of supposed 'extraterrestrial beings' having visited our planet, there is no DEFINATIVE proof that they had done so. Thus, bordering upon extrapolated science for an answer to your concerns,:-
Should they visit us, it will mostly be in probe forms - drones. As our own civilization had advanced physically & mentally, should such probes be destroyed by us, it is only a warning to them that such visitations are NOT invited. Better they show themselves, show their intentions & be the guest to our homeworld, instead of spying on us, as their form of civilization would dictate benevolence & intelligence for their ability for costly interstellar travel.
Furthermore, the closest habitable planets are at least 11 light years away - which means at least 22 yrs of research to finally reach those beings, & will be more, even thousands of lights years away for other civilizations. At the rate of our advances in science & tech, we may already had achieved the status they have as a type 2 civilization & will meet as EQUALS.
However, currently Humanity had not reached Type 2 Civilization status yet, & thus the strife & chaos in our selfish World, with MORE to be done to achieve it. The Human Earth born spying will only naturally continue, more so with drones. Such drones have to be lightweight to stay in the air longer, would carry little fuel or battery cells & may not even be armed to pose a threat. Thus rather than to shoot them down, it will be better to capture them for study, regardless if they are terrestrial or extraterrestrial, to further our knowledge & evolution.