how to block a particular user from accessing to the machine via ssh?
but i still wan't to have the user be able to access the SFTP server witch is a subcategory of sshd

IP blocking will not work as the user might have a dynamic IP address
and giving the user a fake password will not work as well as the user needs the password to access FTP and/or SFTP

I've never done this myself, but from what I've heard the best way to do this is to use [URL=http://sublimation.org/scponly/wiki/index.php/Main_Page]scponly[/URL} (which also supports SFTP).

Is this a regular user with an account on the machine for other purposes? I wonder if sftp would function if you replaced this shell with /bin/false.

this is a regural user without access to machine via ssh
only with FTP or SFTP
if i can't get SFTP to work, i'll just use FTP then, it's no big problem

could i also do a group thats not permited to login through ssh?
so that i would create a group named nossh or something
and set this group as a group to a user, so that every user in this group wouldn't have access to ssh?

edit: if i set a users shell to /bin/false it doesn't work
tried that one...not on purpose when i changes roots shell to /bin/false by mistake

I think that if you deny someone ssh access that you also deny them sftp access, but I'm not certain. The sftp manpage refers to sshd_config. Read through the sshd_config manpage. You can use DenyGroups, but using AllowGroups to allow only the authorized groups may be a better idea. Using this denies all other groups.

There is a "Subsystem" entry but I don't know if these options would allow someone denied with DenyGroups. I kind of doubt it.

It sounds like the "scponly" option is the way to go. That is why why they wrote it I suppose.