Only that which can be demonstrated to be possible, is possible.

You are currently traveling through the universe on a habitable sphere at millions of miles per second. So there's your proof that it is possible for intelligent life to travel through space. Note, I'm not stating that its proof of aliens, but rather that it's proof that an intelligent lifeform could travel through space.

It does mean their claim is false until they present evidence it is true. Look up Russell's Teapot. It is mainly used for the claim of god but can apply to anything in a similar scenario. There's little difference between a person who claims he had a spiritual experience and a person who claims he saw aliens.

Unproven is not the same as false, nor for that matter is it the same as unprovable. You are the only one making a claim here: that alien abduction and alien life have been proven false. they have not. the absence of evidence is not the evidence of absence. A false claim is one that can be definitively shown to be false, not one that cannot be definitively shown to be true (an unproven claim).

the teapot argument isn't even a cogent one. Nobody has suggested that such a teapot does exist, whereas claims of extraterrestrials existing go back millennia. also you are disregarding mountains of evidence in eyewitness accounts and government documented encounters as irrelevant simply because it is not definitive proof (at least to your own reasoning). there's at least as much evidence of alien life in your own genome than there is evidence of human evolution according to the guys who discovered DNA.

Because it's possible that interstellar travel is impossible which would make aliens visiting another solar system impossible.

The possibility that interstellar travel might ultimately be impossible doesn't necessitate aliens visiting the solar system being impossible because it's built upon a hypothetical (as we don't have any reason to suspect interstellar travel isn't possible). Were you to prove that interstellar travel was impossible, this might help your argument inductively, assuming you could also prove that no other intelligent lifeforms live within or near the solar system as well as no intelligent lifeforms can possess the ability to travel interdimensionally

except that some people are retarded

Yeah, that's an ad hominem and does nothing to support your claim. It just makes your argument look weaker and less convincing. Disagreement about the existence or nonexistence of one thing in the universe doesn't make a person stupid or smart, no matter how mundane or ridiculous that one thing is. At best you might argue that it makes one somewhat delusional, which we all are to some degree and on some issues.

With no compelling evidence that even a single of these UFO sightings is the result of extraterrestrial intelligence the only logical conclusion is that they aren't.

Now who is being shortsighted? We have undeniable proof that just about every piece of evidence on the matter has been deliberately suppressed by TPTB at one point or another based on the fact that so much regarding the phenomena has been classified, buried, redacted, and compartmentalized behind special access programs. The only possible way you can make the claim that there is no compelling evidence for extraterrestrial life is by proving definitively what the purpose of such a massive conspiracy to hide UFO related documents while attacking eyewitnesses and spreading what can now be acknowledged as obviously false propaganda against the possibility of ET life.