Clear Open Items

Experts,

I need to clear some open items in a Co Cd as we are planning to start fresh by closing all open items. These items were paid off in a different system, but they were not updated in SAP.

So what are my options to clear these open items :

I am guessing i need to create an account and then clear these open items with that account, but then how do i do that - LSMW ?? as they are a lot of open items.

All answers/options/ideas/views would be really appreciated and duly rewarded with points.

Thanks,

Nandita