First off, everyone would avoid this issue if they did two things
Price your self out of that market:
If you are a bargain computer repair shop and charge little to nothing, then these are the types of customers you are going to get. You need to price your self in the range of customers who are not going to try and get one over on you. These are the same customers who are more likely to be reasonable and understand that you get what you pay for.
Run a FULL diagnostics on every computer regardless of the issue:
I learned to do this a very long time ago and jft135 is really the only other one smart enough to suggest this. If the customer wants to refuse a diagnostics, then refuse their business, simple as that. Trust me, if they refuse a diagnostics, you don't want to deal with that customer anyways.
Other reasons why you should run a full diagnostics with each repair:
Many of the people who come in for a virus removal or a tune-up on their computer end up with a hard drive replacement because their hard drive is failing. Why, because often times the same symptoms can be caused by a failing hard drive. Even if their is a virus, there is no sense in removing it on a failing hard drive.
Half of the screen replacements that come in also have failing hard drives due to the nature of the accident.
And just as suggested before, reasonable people (the ones you hopefully target) will be happier that you helped them prevent data loss or told them about problems before they wasted their money. In fact, if you aren't running a full diagnostics before every repair and informing your customer of issues found before you do any repairs, then you are shady as crap.
Only shady people refuse to run a full diagnostics with each repair
Finally, as far as I am concerned, only shady shops decide not to run a full diagnostics. These shops are typically the ones who will only repair the obvious issues, knowing that the customer will probably be back to fix something else that should have been caught the first time it was in the shop. For instance, if you replace the screen and their hard drive was failing, but you did not check it (or if you did and did not inform them that it was failing), then your almost guaranteed to replace that failing hard drive in a matter of months. However, had you checked that hard drive and told them it was failing, they probably would just opt to get a new computer. Just another shady practice that focuses on you being #1 instead of your customer.
ProTech Support
Once sold a custom built computer to a customer....the person had given me a lot of trouble in the past, so I had her come to the store to sit down and use it before she took it home. She was there for about an hour, than left.
You should have known better and refused her business. Period.
usacvlr
Yeah and then you get blasted for trying to upsell her on other problems that she didn't know about and that she didn't ask to get fixed. Upselling usually just pisses people off.
As mentioned before, you are obviously doing wrong and obviously not pricing your self out of that market.
Austin
Same as above.
jft135
What I have learned, is that I tell everyone up front that everything that walks through our door gets a full diagnostic up front no matter what is wrong with it. This is so that we don't accidentally charge you to fix something on a computer that may have a different catastrophic failure a week from now. We just want to be thorough with your system.
One of the only smart people to respond to this topic.