Hi Brad,
Fist off, backup your database before attempting these steps.
You should be able to purge old transactions. Purging works based on the tab you are in, so you will need to be inthe Command Center and go through all tabs (Accounts, Banking,Sales, Purchases, etc.) to ensure all old transactions are purged.
In layman's term, go to Accounts, then File\Purge. Then go to Banking, then File\Purge, etc.
Once you have done this, you should be able to delete the items you wish.
Finally, we at Accounting Addons have a process to delete customers, suppliers (and i believe Items) in bulk. It essentially sends the keystrokes for you to MYOB. IF you are interested, visit our website at http://accountingaddons.com.au
Best of luck.
Nic