Checking the "Allow b rates" affects what the AP will transmit. In particular it will send most overhead packets including beacons, probe responses, and authentication / authorization as the slow, noisy, 1 Mb DSSS signal. That is bad for you and your neighbors. Do not check that box. The default really should be unchecked.
The AP cannot control what a client will transmit. Clients at the edge of coverage will likely drop down to 1 or 2 Mb and continue to communicate by sending a slow, noisy, DSSS signal. In general the AP has a stronger transmitter than the client, so those clients can continue to receive the AP even though (with the box unchecked) its minimum rate is 6 Mb. DSSS on the air from any source remains bad for you and your neighbors, but since OpenWrt still doesn't have RSSI-based dropping of stations, there isn't much you can do about it...
other than raise the minimum AP rate even higher than 6 Mb. This will make the AP unusable to distant clients, which again is a good thing for public wifi in general. Consider that especially if it is a coffee shop or other small place where legitimate users will be rather close but you don't want moochers from across the street.
Selecting N only instead of Legacy will prevent devices that do not advertise themselves as N capable from connecting at all. It does not affect the rates that may be used after connection is made.