Defuse exploits were discussed in the admin forum a while back and the consenus was that since a lot of people may not know that what they're doing is an exploit, the admin would slap them off the bomb and then educate them that what they're doing is an exploit. Of course once you're warned and you do it again, well that's when the kick/ban should come into play.
More blatant exploits, such as defusing from the sewers in Mill would likely warrant a kick as a warning since if you're doing that you pretty much know it's an exploit.
Essentially it came down to the fact that you have to be able to see some part of the bomb when you're defusing.
Putting it in the motd/server rules would be a good idea in my opinion.