You can do it with cronjob.
37 19 * * * find /path_to_spamdir -maxdepth 1 -ctime 7|xargs rm -rf
If I didn't make any stupid typo or mistake, this crontab would delete any file in the spamdir older than 7 days (after its creation date) at 19:37, every day.
If you have doubt, consult wiki and/or man page of "find" command for more detail.
To setup crontab, you can create a file called mytab or whatever,
and also another one called cron.cgi:
You need to put both file in the same directory (password protected directory).
And you need to set executable permission to the .cgi file.
I think you can do all these in WebFTP.
...* I'd be happier if DH had free shared SSL and redundant server setup. ..*.