Making a user 'read only'

Hi,

I'm looking for a way to revoke all object privileges except SELECT on a user's *own* objects.
Or for some other way to produce the same effect: that a user, although able to log in and read
data from their tables, is unable to write. This is for an 8.0.5 server.

I need to be able to do this at a user level -- so making the database read only isn't an option,
nor is making their tablespace read only possible, as many users share a tablespace.

I thought this would be a facile operation but, unless I'm missing the blindingly obvious[1], this doesn't
appear to be the case.

Cheers,

[1] -- This is far from impossible.

--
Alex Hooper