Perhaps someone will answer this with the magic bullet for you, but the only way I know is described below
The Journal is designed, as far as I know, only for one at a time deleting (for the average user).
The journal entries are kept in the directory store. If you know linux, then you can find it, but the entries are coded files and you won't know what you are actually deleting without doing some more linux digging. If you want to clear everything out, then you can do that and the store directory will be recreated when you do a new boot. If you don't know enough linux to be comfortable doing this, then forget it. It is safer to delete the entries one at at time in Journal.
Journal will delete older entries itself when it fills up and newer entries are added. It's only two mouse clicks, several for each delete, 16-20 a minute, doing it via the Journal. And it allows you to be selective.
If I give advice it is usually because I did it / tested it successfully on my XO. I will warn you otherwise.