Hello, please advise me hot to clean fast all my private data on N900 - contacts, calendar, tasks etc... 10x in advance

Flash it (including the EMMC where most of your data is kept): http://wiki.maemo.org/Updating_the_firmware

any software solutions ?
especially for calendar items and phone logs ?

That was a software solution.

I dont want to flash the device !!! Just to clean calendar items and call logs. I cannot find way to clean these data at all ?!?

You can delete all the calendar entries from under the Settings inside the Calendar Widget

You can do the same for contacts from within the Contacts widget.

I cannot find this option in calendar widget. and how damn it to
delete birthdays ?!? I have only edit option

try a tool documented in my signature... you could potentially create your own customized cleaners

You would have a fresh new device since a couple of hours, if you just have followed the first advice you got!