Hello Srikanth,
Returning false from the tpc:pre-submit handler should be all that is required to prevent the form submission. You could try to use the tpc:validate form submission event to do something similar, although it is likely that custom JavaScript code is the reason that this form continues to submit.
If you would like, feel free to post your code in this thread so we can see if anything looks out of the ordinary, or send an email to support@crmportalconnector.com where can help you continue to debug this problem.
Cheers,
Josh