If you are about to launch a siege or a raid and have the select best Guardian option on and you decide not to send the Guardian, click the box to deselect Guardian then launch the raid, the Guardian is launched anyway.  I was wondering how my Offensive Guardians end up sieging cities all the time... it's because the option to deselect them doesn't work.  You have to not launch the raid, exit the raid screen then return to launch the raid in order for the Guardian to stay home. You can only disable the option on "future" raids but you cannot deselect a Guardian who is already selected for the raid.

I think that offensive Guardians should automatically go home after a siege lands since they provide no benefit defending the siege.  My best 2 Offensive Guardians have been stunned multiple times after being sent in error to hold a farm siege with 1 Javelin. LOL