I was doing the some test on this, and the result was just as the Power BI staff Sun states in this thread.
If you've created a real-time dataset and you didn't check the "Enable historic data analysis" option, then there is no need to manually clear its entries, since Power BI is not storing anything.
If you've created a real-time dataset either by checking the "historic data analysis" option, or via an Azure Stream Analytics output, you can delete its rows using our REST API. You can start with our documentation here, let us know if you have any questions!
Also, as per the online documentation,
There is currently no way to clear data from a streaming dataset, though the data will clear itself after an hour.
I think you can submit an idea at Power BI Ideas and vote it up.