> Buy the site and pay the owner to keep on running it.
I think that would send the wrong signal to would-be copycats. I think the chosen solution, allow them to continue but non-commercially is a pretty good one. If it's a fan site then it should be non-commercial.
IKEA Hackers wasn't just some site using the IKEA name, it was a high-quality and well-established community. If IKEA had purchased this establishment, I don't think a reasonable person would conclude they would give the same treatment to just any other trademark infringer.
And even non-profit sites need money to pay for hosting.
Buy the site and pay the owner to keep on running it. If it's owned by IKEA there's no trademark dilution problem...