If a constituent has an organizational relationship within The Raiser's Edge and that organization is set to match gifts for the constituent, then a matching gift will automatically be added from that organization.

If you do not want future gifts to come over as matching, delete the relationship.
OR
  1. Open the Constituent record and go to the Relationship tab.  
  2. Open the specific Organization relationship.
  3. Go to the General 2 tab of the relationship
  4. Uncheck the box for "matches gifts from XYZ"
  5. Save and Close the relationship record