Developers delete a foreign key constraint when they want to remove the requirement to enforce referential integrity. To remove a foreign key they should follow the next steps:
1. In Object Explorer, right-click the table with the constraint, and click Design (Modify in SP1 or earlier).
The table opens in Table Designer.
2. From the Table Designer menu, click Relationships.
3. In the Foreign Key Relationships dialog box, select the relationship in the Selected Relationship list.
4. Click the Delete button.