What Modicon software are you using?
Wondering whether its a security issue with the PLC or with the software. I don't recall the 984 having a password in the PLC itself, then again I try to remember as little as possible about 984s. But if its the PLC then clearing and reloading should work, but if its a feature of the software, for example PLCWorkshop for Modicon password protects access to functions in the software such as the ability to save or download, not just the PLC program itself, in which case you would be screwed if you erased the PLC program.