I quietly back away from their involvement and do it myself (or find someone that can).
A lot of times, I run into people who really don't know better. I try to go easy- one- because they might really be trying their best, or, two- because I might be the complete psycho and not even know it.
It's a fine line between them being the idiot vs. me being the idiot.
For example- Lowes. I'm looking for a bolt. Didn't ask for help, but someone walks up to help. I haven't found the bolt yet. To be polite, I let them help. They push me aside and go through the same bin I was at. "Nope... sorry, we're out" "Ok, thanks for helping" I say, but as they leave, I stay, sorting through adjacent cubbies where often someone tossed the bolt I need in the wrong drawer. They- employee- might notice me digging on, but I am biding my time in a little extra digging vs. driving to the next home improvement store. That's my level of thoroughness at the local bolt bin.
I deal with this in my profession as well. An entire continuum of people not getting good enough and not even knowing, vs. expecting the impossible and never getting it.