How do we report someone who spams your HA message center?

do you mean your PMs?
assuming you have already tried to settle the issue, I would maybe PM a mod or admin and state the problem.

edit: you can also block individual people from sending you Pms in your control panel.

Indeed. Report the issue to either a mod or admin in a pm.

Please forward offending PMs to either me or Garf or CiTay.

I forwarded it to you zZzZzZz.