He did this last time when I left my laptop at room. Opened the system recovery mode from advanced Ubuntu options and changed the password using 'passwd usrname ' command.Is there anything I can do for this to not happen in future.
For this user case probably the best "solution" is to re-install Ubuntu with full encryption (LUKS). This would secure the data and prevent changing the root/users password(s).
This could be defeated with a few methods, both hardware (key loggers) and software (custom kernels), but both of those options are unlikely. The OP can assess these risks.
With the above suggestion, I agree with the advice that the ultimate solution is to prevent physical access. In general physical access is root access via any number of techniques.
Yes. Set the root password. Open a terminal, and do:
Enter a password. Then the root shell in recovery mode will always ask for this password.
Note that all this is futile as he can boot into a live USB and change your password.
Ultimately, given physical access, the security battle is lost.