I checked the schema tab, types are correct there. But in the explore view, some types are wrong. For instance
How do I tell it those are string instead?
I agree this is a subtle part of the studio. There are two distinct concepts akin to a type. You can see both when editing a preparation script:
Each column has “Meaning” and “Stored as”:
The Meaning is used to infer a storage type when DSS is asked to automatically create a schema, and this is the only case where it's required to click “Meaning→Change” (which adds a “change meaning” processor). Clicking “Meaning→Change” simply overrides the auto-detection.
Meanings are richer than storage type: they detect that a column might be an IP, a country, a gender, an email, a json object, etc.
See also http://doc.dataiku.com/dss/latest/preparation/data_prep_and_schemas.html