If the friendly approach may not work, or you're unwilling to attempt one, then report them to the local MCA Marine Office or alternatively the Training and Certification Section at the RYA.
Let us all bear in mind that many of us spend a great deal of time and money when operating our businesses by ensuring we are complying with the law. Those that don't comply are cutting corners, possibly to save money, which leads to unsafe practices that may and do lead to injuries or even worse.
Check out this link to see what some people have been up to as it makes fascinating and frightening reading.
You may also find this page from the MCA web site helpful as it shows what sort of reports the MCA Enforcement Team are interested in receiving
Incidentally, my company has been involved with the Enforcement Team (someone decided to forge one of our course certificates) and I found them to be extremely helpful and understanding. In fact I felt that they have the same standards as many of us have!
Hope this helps you with any future dilemma