Exception list for wireless possible?

Is it possible to tell my 3GS to not try to connect to certain wireless connections? For example, I want my auto-wireless option to stay on except when I'm at the office because every time I look at my phone to read new mail, it tries to connect to the secure wireless connection in my office. I am trying to find a way to tell my phone to ignore this connection in the future, is that possible and still leave my wireless option on?

Thanks!

 

Interestingly, my screen looks exactly like above but that "forget" button is not on my screen...?