How do I block a particular email id from sending ecards to me?

It is not possible for us to prevent a user from sending emails to your email address.

An easy way out, is that you can create a filter in your mail settings and specify the filter criteria for the particular sender that you want to block.

For example, if you are receiving unwanted emails / ecards from abc@abc.com, create a mail filter in your email program, to move all such emails directly to your trash folder.

For emails : the filter should be to move sender email as "abc@abc.com" to your trash folder;
For ecards : the filter should be to move subject line containing "abc@abc.com" to your trash folder;

Be careful not to block all emails or all ecards as then valuable emails/ecards will not reach you.