I have my own section at work and the guys who i look after are never wrong,regardless,so i find that in the first instance you need evidence to back you up,word of mouth is no good as people tend not to believe you.

Then, when the time is right, i take the said person/s to one side,even if that means walking down the car park out of the way,so i as i dont show them up in front of the other guys.

In a calm,straight to the point manor we talk it out,never raising my voice,then,when it's talked to death and sorted and the guy has been "educated" rather than bollocked,i usually say"consider yourself bollocked,its done,thats the end of it".

This way you get respect and hopefully some lessons learnt.