I would add a "Parameters" Query (a table with a single row - created using Edit Queries / Edit Data) with a column called [Data Load], with a single row containing "Yes".
Then I would add a Filter step to the end of all the other Queries, referring to that "Parameters" Query. The Filter syntax would be:
Parameters{0}[Data Load] = "Yes"
That syntax is a bit obscure - it means:
Go to the Parameters Query, get the value from the 1st row, in the [Data Load] column, test if it equals "Yes".
When you want to empty all the data from the .pbix file, edit the Source step in the "Parameters" Query and change the [Data Load] value to "No", Apply and Refresh.
I've built a working example of this which you can download from my OneDrive and try out: