I studied and believed astrology between the ages of 12 and 16. At 16, I found a few ways to strongly falsify it. The problem is that the readings always end up being somewhat true and people become enthusiastic about the readings. All I did was follow a particular book, but yea that was tough to falsify for me at that age.
In any case, it depends on what you value. If you don't care that someone is into astrology, then go with the flow. If you deeply believe in science and it's one of your core values that people close to you believe in that as well then a short and polite conversation is all I see happening. I'm of the latter kind. I'd have never made such a strong comment though as it is my experience that people aren't really open to change their minds on this (having done readings on a few dozen people as a kid). So why waste the time?
Both "just be yourself" and "learn how to be someone who is capable of going with the flow" are good advice for social situations.
on the other hand, it's also important to 'go with the flow' sometimes, specially around things you don't consider too important; whichever those may be (I've noticed that what is important is even affected by my mood).