Does lying indicate that the trade risk is higher than that of someone who is honest, thus requiring negative trust?
If the case involves money - then you have the right to send him a negative tag as a warning to other users. I agree with what suchmoon said - if he lied on something unrelated to trading, then your negative tags might be questioned. Apart from that - you are free to send tags to anyone as long as you don't abuse the system.
Someone saying that Santa exists or that Earth is 6000 years old is not necessarily a potential scammer.
Maybe he was just kidding - LOL.