The module should help a lot in reducing the amount of spam in the moderation queue. I've been running a module that does something similar but only on selected nodes, with great success. Every now and then someone still calls my bluff, I guess they don't believe me either ;)

As for code feedback;
- there still are a few variable_get()'s without second argument. Even though its unlikely they will ever be needed, the parameter is still mandatory in D6.
- do you really need that global variable?
- maybe pass the message through filter_xss_admin.