Once someone is given (or finds) your wireless LAN's ESSID, and if you are not running WEP encryption, that person can use your WLAN whenever they want. You can block them, however, by enabling WEP, using a non-obvious WEP key, and not giving out the WEP key information. You can also use your AP or wireless router's MAC Address filtering controls and allow access only to desired clients.

Unfortunately, these capabilities have no time-of-day controls in presently available equipment. So you'll have to manually enable and disable them when you want to control access.