There are two ways to accomplish this:

1. The first way is to use the Marketing Opt-Out flags on the member's MASTER record. Introduced in the CU*BASE 9.2 release (August 2009), these are two flags that can be checked to exclude the member from third-party marketing communications and/or CU-related marketing commnunications.  See the "Opt Out" link below to read more about the use of these flags.  When creating a database file using Member Connect tools, or the CU*BASE List Generator, or even the CU*BASE Report Builder (Query), you will simply set up selection parameters that exclude members who have the applicable opt-out flag checked. 
2. You could also take advantage of User-Defined Code fields (formerly referred to as Mail Codes) on the member's MASTER record.  The codes can be configured via Tool #934 "User-Defined Code Configuration" and added to specific memberships via Tool #15 Update Membership Information."   These can even be used in combination with the other opt-out flags for more complex needs.

NOTE: Traditionally when Client Services creates third-party marketing files for our clients, we exclude memberships with anything other than a zero (0) in these two fields as well as members with bad addresses.  We will also begin asking about how you would like to set the marketing opt-out flags as well.