Many spaces in our production sites are for test purpose (sandbox). These spaces are readable by every user authenticated.
The sandbox spaces comes always on top list when searching for something, because they are used a lot.
I would suggest to make an option on the Confluence admin indexing panel (see the attachment) to turn on/off the search indexing for space(s).
It would be easier to select all the spaces to be disabled or enter the keys...

The capacity of Confluence to deal with the search can be affected as the follwing scenario:

I'm searching for the word "house". When the search returns all the hits:

solution1: Confluence hides all the hits from Sandbox spaces - Lucene still index all the spaces but Confluence hides the sandbox spaces.

solution2: Lucene has never indexed the Sandbox spaces - Lucene has been commited to not index the Sandbox spaces.