With Other Actions --> Delete, if you create a table view to show 900 servers a a time, that may go quicker, but the processing will take some time, but the amount of clicks would be less (If you do choose this method, please use Chrome or Firefox, not IE)
Alternatively, you can use Java SDK for CCA to tap into the delete server methods
Even deeper, what do you want to delete, just the discovered data from NDG? If so, there is a property in CCA UI --> Administration --> Configuration --> Properties --> group="ndg" --> name=" comm.attributes.prune.interval and comm.relationships.prune.interval; . Set those to 0 (or 1, don't remember off top of my head); this will delete everything out after 1 day (assuming you have 2 days to wait (2 because if you set it to 1 today; you have to wait to the end of day tomorrow to have the data expire and the following day, all data should be blank). Now, if you choose to do this, you cannot set that low of a number in the UI, you will need to modify it directly in the database in a table acm_prop. Search for the same names as those properties and can set it 0 or 1 there (I think 1), then recycle the CCA Server service