My guess would be that you've got non-customers who haven't configured their computers to attach to a specific SSID, so they're trying to attach to the strongest signal they hear, which is you. There aren't many ways to fix that, if the owners of those computers won't fix their config.
You could always add their MAC to your connection table, but firewall/drop the traffic from it. I ended up doing something like that once to deal with a business near our NOC that didn't want to configure their laptops to only connect to their own local AP's. It stopped the thrashing on our AP and they eventually starting configuring their laptops correctly just to get them working again.
I tried that with no effect. Put them i ACL with no encryption etc with no result. Also the rate (several per second) isn't normal for any kind of client trying to connect.
/Henrik