Why can I drop table when logged in as not owner? I read that only owner can drop table and no other user may be granted to do so.

I created a table with owner postgres and then drop it as another user. What privileges should I revoke to disallow it?

The user has all the privileges granted by default in db properties but I don't see any "create table" or "drop table" privilege.
Superusers are always exempt from lowly permission checks. The documentation:

superuser status
A database superuser bypasses all permission checks, except the right
to log in. This is a dangerous privilege and should not be used carelessly; ...

To see whether your current user is a superuser:
SHOW IS_SUPERUSER;

To list all superuser roles:
SELECT rolname FROM pg_roles WHERE rolsuper;

And the schema owner can do it, too. The documentation for DROP TABLE:

Only the table owner, the schema owner, and superuser can drop a table.

To list schema-owners:
SELECT n.nspname AS schema_name, pg_get_userbyid(n.nspowner) AS owner
FROM   pg_namespace n
WHERE  n.nspname NOT LIKE 'pg_%'
AND    n.nspname &lt;&gt; 'information_schema'
ORDER  BY 1;

About the same as \dn in psql.
And who are you?
SELECT current_user, session_user;