Hi @scnz and welcome to the Community!

Here’s one possible way you could achieve this use case:

1. Create a new flow from scratch:
    

   

2. Choose the Segment Trigger options and select the Segment you referred to above:
    

   

3. Select and drag the Update Profile Property block into the flow:
    

   

4. In the Block Configuration menu, select “Remove Profile Property” and “Last Name”, as shown here:
    

   

5. This last step is very important if you want to ensure that the entire Segment is collected in the flow. In the upper right-hand menu, navigate to Manage Flow > Back-Populate Flow Recipients:
    

   

   In the Back-Population window, select the second option, which will treat all of the members of your Segment as if they all joined at the same time, so that all of their profile properties will be updated simultaneously:
    

   

Now, simply click the Review and Turn On button, and turn your flow live. Now, all of the members of this Segment (in addition to future members) will have the Last Name property removed.

I hope this helps, and thanks for using the Community!

- Brian