I'm not really trying to be secretive but there are several details about my business you would need to know to understand my specific "selective denial" method (as strategy consultants call it). But the general gist is this:

Many startups are so desperate to find customers that they are even holding on to the ones that cost too much support. (Note that in a startup support cost often takes away valuable time from the founders and thus is more an "opportunity" cost than a monetary cost, they should have spent that time growing the business with more valuable customers).

I chose to do only e-mail support because it self-selects out a type of customer that wants to use the phone (also phone support scales poorly, especially when expanding internationally). There are ways you can encourage the "dummy" user to desire phone support, and then gently point out that there are _other_ providers that have that. If you do this properly the dummy client will actually feel they've had good customer service (and you could argue that they have, we just weren't meant for each other) and your over eager competition gets stuck with the dummies.

Edit: I thought I should mention, for a startup it can often be unclear which customer is/will be valuable and you can learn a lot from the "wrong" ones, so be careful with "selective denial" unless you've run the numbers and are sure what you are doing