Currently the only way to hide them would be to remove READ permissions from those spaces. The simple search uses the Alfresco Lucene indexes which covers all indexed nodes that the user has at least READ access to - no matter where they are in the repo.
Having the simple search restrict to the 'current' space sounds like a valid suggestion for a configuration option. I have added it as a JIRA item: http://issues.alfresco.com/browse/AWC-1251