Clear your memory card safely

Deleting a file in Win/Linux is almost like locking in a safe. It can be  retrieved from the drive easily with a recovery program as long as it is not overwritten by some other data.
Even quick format also won't delete permanently.

In Linux we have the dd command to clear everything in the drive to zero. It is kind of resetting the drive.
Note: It will clean up all the file-system related information in the drive. So after cleaning the drive make a file system in it with mkfs.vfat or mkfs.ntfs, etc.

dd if=/dev/zero of=/dev/sdc bs=1M

To know which file represent your drive run dmesg. which prints the device file details at the end.
For me it was

[11540.353227] scsi4 : usb-storage 1-2:1.0
[11541.353843] scsi 4:0:0:0: Direct-Access     Generic  STORAGE DEVICE   0902 PQ: 0 ANSI: 6
[11541.358108] sd 4:0:0:0: Attached scsi generic sg3 type 0
[11541.629445] sd 4:0:0:0: [sdc] 15523840 512-byte logical blocks: (7.94 GB/7.40 GiB)
[11541.630924] sd 4:0:0:0: [sdc] Write Protect is off
[11541.630934] sd 4:0:0:0: [sdc] Mode Sense: 21 00 00 00
[11541.632410] sd 4:0:0:0: [sdc] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[11541.651727]  sdc: sdc1

So mine is sdc.