Ok, I think it must be the Distribution Agent that is set to run continuously because i've just gone through setting up Snapshot Replication between a couple of Test databases and the options that you mentioned are presented for the Distribution Agent.

However, I can make changes to the tables included in the Publication regardless of the frequency with which the Distribution Agent runs and that is what I expected because the Distribution Agent is unaware of any changes to a Publication - it just pushes (or pulls) the published articles to the Subscriber after a new snapshot has been generated.

What error do you get when you try to modify tables on the Publisher?