I have olimex a20 with debian jessy rel 10. But in one time I run my olimex and there start Android 4.2.2. HOW it can be.
update: It start not from SD. It start from nand, but i don't install it.
how delete it?
Use either nand-part or fdisk software to format the flash memory. These should be available in the image (no need to download).
Alternatively, if there is a nandinstall script in that release you can use it to install the Debian to the NAND memory instead (thus overwriting the Android).
As a side note I highly recommend you to check the wiki article of your board and download the latest official Debian release.
Best regards,
Lub/OLIMEX