While I was complaining about having to deal with their inability to execute their build properly, my main complaint is when I tell them point blank vtec isn't working and they have a boost leak, they still just whine their car is slow and think it's the tune. How do you handle that.
I will usually say "This is what it's doing, and this is what it's supposed to do. Here are X, Y, Z reasons why it's probably not working/is leaking/is broken/whatever."
I have no problem telling people their shit sucks and they need to fix it properly before they bring it back. In your case, re: duct tape fix, tell the guy exactly what you feel is the proper solution and if it's not like that when it rolls back in, send him on his way until it is.
People will whine and complain, but that's not your problem. If they want to go somewhere else and not fix their issue, then they can be a headache for someone else and not cause you any more stress. Either way they either fix it to your standards or get lost, it's win-win.