Due to the relativisitic effects of travelling anywhere close to light speed, interstellar travel is a practical impossibility due to the length of time required. Therefore, there are no space aliens visiting us. Therefore, people seeing space aliens are hallucinating or are seeing manifestations from the spiritual realm. Since these space aliens generally tend to lead people away from belief in Christianity and even towards worshipping and communing with these manifestations, I conclude they are demonic.
An exhaustive list of possibilities, no doubt.
I'll go with the hallucination part. The "evil spirit" thing sounds as whacked as the "little green men" thing -- and for pretty much the same reasons.
"Due to the relativisitic effects of travelling anywhere close to light speed, interstellar travel is a practical impossibility due to the length of time required."
Nice.
Physics from the early 1800's.