Is it possible to make database owner read only? The database owner can revoke certain privileges from themselves such as INSERT but is it possible to revoke further privileges such as DROP for read-only operation?
The privilege to DROP an object does not exist as such and thus can't be revoked. Owning an object implies the right to drop it, and having the right to drop an object implies owning it. Here's a relevant excerpt from GRANT's documentation, which is pretty explicit about this point:


  The right to drop an object, or to alter its definition in any way, is
  not treated as a grantable privilege; it is inherent in the owner, and
  cannot be granted or revoked. (However, a similar effect can be
  obtained by granting or revoking membership in the role that owns the
  object; see below.) The owner implicitly has all grant options for the
  object, too.