This is already built into exim.time limite for auto deleting.
exiqgrep -z -i | xargs exim -Mrm
Sorry, made it sound worse than it is, we limit all users so its never more then a couple of pages, still a button would be more useful then select all, delete, and that for 3 or 4 pages. Sounds simple to implement and gives the finishing touchWith thousands of spam emails in a queue you will run into numerous timeout issues, so you'd better use SSH to clean queue or use a simple script to clean queue with cron.