Go to System Preferences
Highlight AirPort and click Configure, or Advanced in 10.5?...
Choose “By default, join: Preferred networks”
Select the wrong access points and Remove those access point swith the minus button.
Go back to the “By default, join:” page and click the plus this time to add your access point. Enter the correct password, save, reboot, or...
Instead of joining it from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.