Hi, thanks very much for your question. My name's XXXXX XXXXX I'm going to assist you with it.
If a solicitor is threatening you in an inappropriate fashion, this is considered to be professional misconduct and you are absolutely entitled to make a complant.
In order to do this, you will first need to contact the firm directly and use any internal complaints procedure that it has in place (advising them in writing that you are prepared to escalate the matter if they do not address your issues to your satisfaction). If this does not resolve the problem, you can complain to the relevant professional body that governs the solicitor's practice. This will depend on where the solicitor is registered.
If he is registered in England or Wales, the relevant body is the Solicitors' Regulation Authority (SRA): http://www.sra.org.uk/consumers/problems/report-solicitor.page#when-report-sra
If he is registered in Scotland, the relevant body is the Scottish Legal Complaints Commission (SLCC) at http://www.scottishlegalcomplaints.com/making-a-complaint.aspx
Please don't hesitate to let me know if you would like any clarification on the information above or if you have any other questions. I would also be very grateful if you could take a moment to rate my answer.