[Factual stuff]
There are some spam bots that use technology such as this to get around the text-images. You can use a block of interlaced multiple words or logic problems (also see that site for results; more effective).
[/Factual Stuff]

[Hypothetical Solutions]
I don't maintain any public/private fora, so I'm not sure if this is posible, but since I imagine it's mainly for communications within your team, ask for a one-time "code" when signing up such as the name of your robot or a passphrase you distribute at meetings.
I would think you could also restrict the fora by IP address (or by range for folks that use ISP's like comcast).
[/Hypothetical Solutions]

Hope this helps,
Mike