The N900 is different to symbian devices, as it doesn't have the same messaging structure.
There is no such thing as an inbox, drafts, or sent box in the N900.
Instead your SMS are held in a conversational thread.
3. Conversations (top of page, with the drop down arrow)
4. Delete conversations
From there you can select which conversations to delete.
Obviously this will mean your SMS's will be deleted, but it might solve your problem.
Otherwise, I would suggest performing a software update.
You can do that from NSU, found here: https://europe.nokia.com/support/download-software/device-software-update
Select your device from the list, download to your PC and install it.
Then follow the simple on screen instructions.