Hi Simran,
If you're running the report from the portal and wish to hide the refresh button on the CR Viewer, you'd need to contact the maintainer of the portal.
The refresh button cannot be hidden on the CR Designer.
To hide the prompt, as a workaround, you can insert the report as a subreport in a blank main report. Then, using a formula with a hardcoded value, you'd need to link it to the subreport's prompt.
-Abhilash