If you do not want a record to be created (by record here we are referring to a grouping of fields such as an attribute, an address, a phone number/email address, an action, etc.), then you need to suppress a key field. Generally, key fields are the fields required for all Raiser's Edge users and you can find a list of them here.
For example, if you want to suppress an attribute for all existing constituents, you would want to suppress the category and/or the description as those are the key fields for any attribute.
Beginning in version 5.2, you can also use Area Settings to add records only to new or existing records (this is available for areas such as Attributes, Gifts and Solicitor/Canvasser).