If a user is using a proxy, tor, or sock or any combination thereof, this wouldn't really matter as once you have that IP you would have to find out where it came from and so on. If they used a combination of any of those, well, you might as well forget it.
Even if you do get an IP address it may take a court order for the particular ISP to give out who the IP is actually associated with (different laws for different countries).
But if you're really determined, first find out who the domain is registered to. If it's a private registration, which most are, you'll have to do some more digging.
But as zoonini stated, if you're talking about a WordPress.com site, then you'll need to contact them as she previously mentioned.