Mail has a feature, accessible from the Mailbox Behaviors (or Special Mailboxes) tab of the Accounts preferences, that can automatically delete old spam messages from the Junk mailbox. You can use this feature if you tell SpamSieve to put spam messages in the Junk mailbox instead of the Spam mailbox.
(Note: If you do this, be careful not to use the Junk and Not Junk buttons that will appear in Mail when the Junk mailbox is active; you should always use the SpamSieve - Train as Good and SpamSieve - Train as Spam commands instead. You can use the Customize Toolbar… command in Mail’s View menu to remove the Junk button from the toolbar. Also, if you select a message that SpamSieve has classified as spam, Mail will show a banner saying that you marked it as junk. Ignore this.)
To tell SpamSieve to use the Junk mailbox, open Mail’s Preferences window and click on Junk Mail. Make sure that Enable junk mail filtering is checked. Select Move it to the Junk mailbox. If Mail asks whether you want to move all the messages to the Junk mailbox, say No. Select Perform custom actions. Then click the Advanced… button and edit the rule such that the conditions don’t match any messages. For example, use these two conditions:
Click OK to close the sheet. Do not make any further changes to the Junk Mail preferences.
Go to the Rules section of Mail’s preferences and change the SpamSieve rule to move the messages to the Junk mailbox instead of the Spam mailbox. Finally, choose SpamSieve - Change Settings from Mail’s Message menu and, when prompted, say that the name of your spam mailbox is Junk.
You can set Entourage so that when it quits it will delete all the spam messages that are older than 7 days (or whichever interval you choose):