Portal users can unlink records without deleting the record by clicking on a "-" icon when hovering over a record. There are 2 ways to set this up.
1. In the Portal Table
Go to:
Portal Tables
Click on a table in the list
Views section
Turn on the Allow users to unlink records toggle.
Note: This option is only available if you select List or Gallery view.
2. In the User Profile
You can also allow the portal users to unlink themselves from records by adding the linked records field to the child form of the user profile.