In the form properties set it disallow additions, deletions, or changes. Probably something like this (notice that we make the form a snapshot type of recordset too which is probably overkill because that should also make the form "read only"). By setting the additions property to no you should also not get any "new record" options in the form. If you set Allow Edits to Yes then users can edit (make changes to data) but not add new records.
It's actually somewhat unclear how a user can search for a record that then adds it to the database. One would think if the record doesn't exist it would not be found, hence there would be nothing to save. But overall, it sounds like you need to set up the form so that it doesn't allow additions.
<img alt="form properties" src="http://northernocean.net/etc/mrexcel/20110417_formproperties.png" />