Clean cookies before to run a report .... ???

I developed a PDF Report that run smoothly now .
It start with a button and takes a global variable as parameter for the select.

If I clear and delete cookies before to run the report (1) it works and takes the right record .
If I run it once again changing the record and pushing again the button or run report(2) then it make the same report of before … I mean report(1)

Is there a trick to clean cookies or is it sufficient to clear the global variable on script init ??