Here are the text message commands for blocking and un-blocking:

• Block a user: block (their Yahoo! ID) ... Example: block dairyman88
• Block everybody: block all
• Unblock a user: unblock (their Yahoo! ID) ... Example: unblock dairyman88
• Unblock everybody: unblock all
• Reset a user: reset (their Yahoo! ID) ... Example: reset dairyman88

Depending on your carrier, send the command from your mobile phone to:

All U.S. Supported Wireless Carriers—92466
All India Supported Wireless Carriers—58242
TMTouch—17776
Singtel—428801
AIS—83140
DTAC—192466600
Globe—2258800
SMART—258600
Telkomsel—4246000