Ignoring a topic does not make sense. There are hundreds of topic created everyday. None of it should bother you unless your notification for a specific topic is enabled.

A topic is a single entry which create many response. If you don't subscribe for notification then it's unnoticed

We ignore a users so that we can not see their topics, posts (multiple entry is closed by one ignore button)

A board / section is another single entry which consist of multiple topics. If someone don't want to see these multiple entry from a specific board/section the a button to press ignore also make sense.