calibration/validation sets

Hi; 
Does anyone know how to create a calibration and validation set from a particular dataset? I have a dataframe with nearly 20,000 rows! and I would like to select (randomly) a subset from the original dataset (...I found how to do that) to use as calibration set. However, I don't know how to remove this "calibration" set from the original dataframe in order to get my "validation" set.....Any hint will be greatly appreciated. 
TT