I was doing it in a macro as part of this all purpose accept macro that I dug up in the forums here somewhere. This thread and the other thread you posted in both mention the problem, so I decided this one was closer to my needing to use it in a macro. I had not run into another thread mentioning it in relation to the all purpose accept macro.
Code:
/script AcceptGroup(); 
/script AcceptQuest(); 
/script AcceptTrade(); 
/script RetrieveCorpse(); 
/script Repopme(); 
/script ConfirmAcceptQuest(); 
/StaticPopup_Hide( "PARTY_INVITE" );    ***Taking out the word script stops the auto decline, but it do not hide the box, also if you leave out the beginning /, it comes out in whatever chat you have selected currently***
/script StaticPopup_Hide("QUEST_ACCEPT");