You have a few possibilities.
One, you can get a wireless access point that can be configured to specifically block out wireless clients from seeing each other. Expensive, you have options from houses like Cisco that are as flexible as putty and complicated as space shuttles to do this. But once you get them set up you should be good to go. This gets you a setup like you see at Starbucks/B&N/etc.
Two, set up a firewall system that your protected systems sit behind. On the other side is a switch with your public wireless AP, and have a block of your IP reserved such that your firewall blocks all incoming traffic from that IP block. Cheap, not all that elegant, and requires documentation for maintainers. But you implied you're running a small business so it's possible you could get away with this arrangement.
Three, shop around for a SOHO router that specifically blocks out wireless from wired. This has the disadvantages of A) blocking your own wireless from seeing the wired side and B) any SOHO router, in my experience, tends to go flaky after a year or two, so it can get to be a pain in the arse to keep buying a similar unit as backup for the first unit's failure and you need the specific feature set (I eventually bought a separate wireless AP and a switch that was just a switch so I could reduce problems on my home network.)
For a small shop, I'd probably go with the firewall solution. It gives flexibility and if you're a small shop that likes having the techs learn things from good projects, it gives experience in configuring such a machine and learn a little about routing and blocking rules as well as information on how your network works. It also takes a little planning in how you want to carve your network block and arrange to have things handed out (since if you're blocking traffic in and possibly out of your protected network to the block of unsafe systems you'll need a way to hand out IP's to the unsafe systems that is separate from your internal DHCP server, essentially creating two networks that coexist in your overall allocation block.)