mail abuse@whatever the company is and they are obligated to act upon any abuse being effected, failing that report them to abuse@whatever thir isp is and if it does not stop the isp is obliged to pull the plug on them.
Slysy, this may be something for the police to be interested in. I would think the slanderous messages are a civil thing. Maybe someone knowledgeable on this forum could help you there. I would think though that abusive and threatening emails would be something for the police to act on.
It is unlikely that the police will show much interest, since they would consider it a matter for civil litigation. Best to close your guest book and contain the abusive messages for as long as it takes. Any form of litigation is costly, and destructive in time and sanity. Eventually the abusers will give up. Inaction on your part may not be very satisfactory as far as seeing justice being done; but in the long term you will come out on top.
ah but (a) is it from the company director for certain? (b) you do not know if they do not have abuse@ facilities (c) if you look at the full text of the mail there is a good chance that that somewhere there you will find the isp, there is a fair amount of assumption in your thoughts, might be best to gather factual evidence or you could be the one defending your actions (just a thought) johnny.