I've been granted a 3-month B1/B2 visa for the US and now I can't go. Will having an unused visa in my travel passport have any consequences like future visas refusal? Do I need to do anything special?
Not at all. A visa is merely permission to turn up at the border and be evaluated for entrance (ie make a formal request to enter) to a country.
However, you've paid for it, you've got the stamp, but nobody is waiting at the border for you to arrive, or expecting you. If you show up, you show up, if you don't, you don't.
If you're really nitpicking, one could argue you're missing an opportunity to visit and show that you're a reliable visitor - ie establish a history as a trustworthy tourist, but not going is not going to weigh negatively at all. It's just a monetary waste, and an opportunity missed.
However, don't forget to cancel your flights, accommodation, insurance and any other things you might have booked in advance.
Well, you should have a genuine reason for a) Applying for a visa and b) Not travelling after receiving the visa. People's agendas change and there may be perfectly valid reasons for why you did not go. The visa officers will definitely question you on why you did not go—when you apply again. Make sure you have a legitimate reason.
My little advice when dealing with Visa officers
These folks are well trained and will certainly be able to identify if you try to lie. Therefore make sure you do not lie. Just be frank and explain to them.
In addition, arguing with an officer will only make things worse—even if you are not granted a visa!