Is there any way i can disallow an ftp Login to change current working directory??

Yes, with the proper restricted environments using utlities like chroot. You basically do not give the user any ability to cd or to execute commands outside of a very restricted setup.

I just have one more doubt,
I want to diable Telnet for an user , but that user must be able to ftp ....
Wat are the choices i have ...
I tried to stop it in .login . But problem is it logs in using X-server .

1) Can i specify a shell as /bin/false ... so that it cannot login .
Will this work for both X - login as well as telnet ..

If you are using tcp wrappers (tcpd), you can
control what users are allowed to use what
services individually.
Check out the man page host_access(5)

For instance in /etc/hosts.allow...

in.telnetd: ALL EXCEPT jsmith@some_host
in.ftpd: ALL

This would allow Telnet for all EXCEPT user jsmith
and FTP to all. However, you should not get
carried away with user lists since it will
cause a lookup to occurr. Obviously, you also
would NOT want to open these services to "ALL"
so I would recommend further restricting this
by using "LOCAL" or even some IP range that
maps to your internal network (or sub-net).

Watever answer u gave is acceptable for telnet .
But i also do not want the user to login using X11 .. ie. On the console or on a direct terminal

For X11, I would recommend using an authorization
file for X. See xdm(1) and Xsecurity(1) for
details on this. Essentially, this sets up
access control lists for X-Windows.