Hi Kannan,
There is a block field updates option in Admin.
See the following article for more infomration:
https://community.marketo.com/MarketoArticle?id=kA050000000KypCCASThis will allow you to block updates on a per field basis.
To block updates flowing from Marketo to SFDC then you should use field level security in SFDC to make sure the Marketo sync user has read only access to the field(s) in question.
I advise taking careful consideration and asking yourself why you would not want existing leads to be updated, blocking updates may lead you to miss out on more accurate information from your leads.
Regards,
Cathal