How do i block one particular number that keeps phoning me up ?
You need to use a phone which has a number blocking feature or a separate call blocking/screening device.
Depending on how persistent your unwanted caller is, the above might not help you though. The caller could simply phone using a different caller ID for the call.
The best approach might vary depending on whether the unwanted call is from a business, an individual or an unwanted scam/nuisance caller.