I have heard from several this one complaint. Their guest network is open but require login through a captive portal page. Yet 90% of the devices consuming an address never log in through the portal page. These devices are consuming lots of leases and as the network administrator you are running out of available leases. In the city environment many of these are the drive-by and walk-by devices that have no intention of connecting to your network.
What to do? I would like to hear from you but will give you some of my suggestions.
Keep the lease fairly short for guests. Many set the lease time the same as they would for an enterprise client. I normally suggest 1 hour but I learned at one customer the other day they have theirs set to 5 minutes.
Drop the 1 and 2 mbps data rates from the 2.4 GHz band. This forces the device to be closer to the AP before it can associate.
Enable and set the Local Probe Response Threshold. The right value requires some knowledge of your AP coverage area. -75 is where I suggest to start. Like dropping the data rates noted above the client must be fairly strong to actually associate. Use this one with some caution as you may have areas where a valid guest may be in a minor coverage hole if you set it too high.
And this one comes with lots of hesitation by many. Actually change the guest network from open to one that uses a PSK. In this model I suggest making the key extremely easy like matching it to the SSID name. You will of course need to advertise the key to valid guests through some mechanism like small signs in conference rooms or other public spaces.
So what are others doing to fight this problem?