Delete record in Aerospike?
First of all you should not be mounting any file system on the device that Aerospike is configured to use. Aerospike manages its storage directly. The file system metadata will corrupt Aerospike storage. 

In storage engine device configuration Aerospike will use the entire device irrespective of how much data you actually inserted. If you do not want to give entire device you can create raw partition on the device (without any file system) and configure the raw partition as device in Aerospike. 

To see how much of the device is being used, you should be looking Aerospike stats. To wipe the device you can either use the dd command or the more time efficient blkdiscard.