Hello, my name is Jim and I am a qualified lawyer happy to help you this evening.
The comments are taking place in the UK so you should consider sending the culprits a cease and desist letter to warn them of your intentions to apply for a civil injunction unless they stop (and give them 7 days to stop their conduct).
A template “cease and desist” letter is available at www.lawdepot.co.uk. You can use your own letter if you wish - it just has to make it clear this is a cease and desist letter, that you demand they stop their conduct, and that if they do not within 7 days you will apply to the court for the injunction and seek your costs.
That letter may well be enough to stop the conduct - as an injunction is a serious legal remedy. If they do not stop then you may have to instruct a lawyer in the UK to make an injunction application for you. The application itself costs £308 and would be made under Section 1 of the Protection from Harassment Act 1997. Harassment is defined as "unwanted conduct designed to cause the victim alarm or distress".
Perhaps the most effective method would be to get a law firm to write to them but you would need to know the client's full names and addresses.
If you choose this option I recommend http://www.stephensons.co.uk or http://www.slatergordon.co.uk
I hope this helps - if you can please accept the answer and give me a 5 star rating (the top right of your screen & then click “submit”), I can answer follow up Q&A's at no extra charge and I will be credited for helping you today.