Leila,
in SQL Server 2005 you can use -EXCHANGETYPE = 2 parameter to limit the data
flow to the publisher, and subscriber database permissions to prevent data
alterations if this is what you require. Actually in this case the merge
triggers still fire and record all the changes on the subscriber into
metadata tables. In SQL Server 2005 this is more elegantly implemented using
sp_addmergearticle @subscriber_upload_options = 1 or 2 depending on your
requirements. Note that in SQL Server 2005 this is on a per article basis,
which is what you request. In SQL Server 2000 the -EXCHANGETYPE parameter is
on the subscription itself, so you will need to create 2 publications and
have it work that way.
 Cheers,
          Paul Ibison SQL Server MVP, www.replicationanswers.com
            (recommended sql server 2000 replication book:
            http://www.nwsu.com/0974973602p.html)

With SQL 2000 you can only select this level of bidirectionality on the
publication/subscription level. So if you want to be able to toggle certain
articles as read only on the subscriber or publisher what you have to do is

a) create a separate publication for them.
b) carefully apply permission to the account you are using for each
individual subscriber. For example lets say you use the account Leila. You
want table1 to be full, but table2 to be read only, or lets say update only.
You grant permission to Leila in the publication db for update only.

Then when you create your publication and get to the specify articles
dialog, click on the browse  button to the right of your table name, and
select the Merging Changes Tab, and then select which DML type activity you
want to be enforced. What will happen is that if you select all three lets
say, and the permission for the Leila account does not have rights to do any
DML on the authors table (lets say), all DML originating at the subscriber
will be kicked back as a conflict.