The only safe way I know of to delete requests is to use the Search Requests page to list the requests you want to delete. You can then use the Select All button followed by the Delete button. It is a manual process that can be a bit painful and you will have to watch that you are not deleting requests that you want to keep.
Deleting contents of tables is very risky as the table structure is rather complex and it would be easy to miss stuff or delete extra stuff. Note, too, that the table structure changes with the release level, too.
We upgraded from 6.0 a year and a half ago so my memory is faded on how this worked. I believe the search request page did not change a lot, though. My suggestion would be to run a search for all requests opened or last modified in a specific time window, select the ones to be deleted and delete them. We used this basic method to delete open requests when refreshing our preproduction instance from production.
There is a specific access grant you need in order to see the delete requests button. I believe it is Manage Requests.