The Access Point (AP) emits a radio frequency that is sent at 2.4 ghz. This actually works just like cell phones, televisions, and radios (AM, FM). You can reduce the power at which the frequency is set. But if the area in which the AP is deployed is not large enough, the frequency (cell phone) actually leaks leak through walls.
Note: You cannot restrict the radio frequency coverage area by limiting the power at which the frequency is sent.
If the reason to restrict the radio coverage area is to prevent wireless clients to access the network, we recommend to implement security mechanisms to achieve this goal. Refer to FAQ on Cisco Aironet Wireless Security for more information about security.