If you're going to be in the service business, you need to accept that or you're going to hate it and be lousy at it, both at the same time.
I hate to say it, but I've been in many a nonprofit/government agency where I've been greeted with just this kind of sign. It's meant to be helpful (I think), but comes across as your organization telling me that I'm an annoying idiot before I even hit the front desk.
In many agencies, this will continue to be communicated to me throughout the building, from the cubes of individual staff members to the conference rooms and public areas. I find that these kinds of signs are notoriously present throughout human service organizations where there seems to be a particular concern about controlling "undesirable behavior." The thing is, it communicates that 1) you expect that there will be undesirable behavior that needs to be controlled and 2) that you don't have a lot of respect for most of your customers.
When I work with clients one of the first things I do is take a look at the signs and other customer communications they have in their buildings. More often than not, I find the approach in Seth's example. It's interesting that when I point this out to clients and offer alternative ways for them to state some of their information, they're taken aback by how they've looked to this point. They just never thought about it.
So here's an idea. With the above sign in mind, walk around your organization and see how many signs like it you can find. If you see them, take them down now and start working on something that's more customer-friendly. If you don't find any, pat yourselves on the back. You're communicating to your customers that you respect them.