Unless you use different Postgres users for each connection (see last section of this page), all users will always be able to query, edit and delete all data from all schemas/spaces. You should also consider that this kind of software allows by definition "SQL Injections", since a core principe of the software is to allow users to write SQL in various places of the application, and therefore it is always possible for an attacker to view, edit and delete data which he doesn't necessarily see through other views.
Users need permissions to create new queries, but, using existing queries, they can use SQL tricks such as filtering (which is done in SQL, hence they can specify their WHERE clause, therefore they can call a stored procedure, etc) to access data from other spaces or schemas.
See our Known limitations.
Spreadsheets and Queries
Spreadsheets and queries are local to a Space, so they inherit the same permissions.