I want to remove/truncate the email reply block from an email. Is there any standards of how email reply block should be? Or is it upto the email client how to format reply block? I tried to google but returns with no luck. Thanks heaps in advance.
There are no standards here, much to the vexation of email users since SMTP was ratified and Usenet was young. Time has not improved this. There are two major defacto conventions regarding quoted email, though.
The first is what Outlook uses by default, which is to format email like a stack. Later messages are prepended to earlier messages, which allows people coming in late to a conversation to read up the message to get caught up in that thread. People who do this are roundly despised by digest-subscribers to mailing lists, and pretty much nowhere else. This is called "top posting".
The other method is called "quoted interspersed", which uses a non-alphabetic character in the first column of the line followed by a space to indicate quoted content. Nesting quotes are generally, though not always, indicated by repeating that non-alphabetic character to indicate the depth of the nest. Complications occur when different email clients in a thread use different characters. The ">" character is traditional, but not a standard (personally I've seen the following used at various times: ] [ * | =). Replies are done to specific questions within the quote block as a way to provide question-threading inside the email itself.
There is a variation of "quoted interspersed" that uses the same quoting markup, but doesn't intersperse replies, instead putting all new content at the bottom of the message. This is called "bottom posting", and in my experience is the defacto standard for FOSS mailing lists. People who do this were once upon a time roundly despised by subscribers using mailers with limited display lines (say, under 30) as it required paging-down to get to the new content of every message. Mailers are smarter about this these days, so the hate has decreased.
In this modern era of HTML-everywhere, the Google standard is to provide a colored line on the left side of the reply block to indicate quoted content and then collapse the block when not needed. When seen in ASCII mode, Google uses ">" style formatting. The gmail markup convention supports both quoted-interspersed and top/bottom posting.
But to answer your question, yes, it is up to the email client to figure out how to markup reply blocks.
What exactly are you referring to? Are you referring to the original message text being in the reply? If so, I would think most email clients have some options for what to do with the original message when replying. Microsoft Outlook has several options.