In the web interface, go to Administration/Scheduler (Custom 1 to 5)

Turn WiFi on:

wl -i ethX radio on

Turn WiFi off:

wl -i ethX radio off

with X = 1 (eth1 for 2.4 GHz) or X = 2 (eth2 for 5 GHz) [at least for 95% of Freshtomato routers]

Example:

wl -i eth2 radio off

Restart the wireless service, both 2.4 GHz and 5 Ghz radios:

service wireless restart