Removing a Given Key from a Groovy Map

req.getParameterMap returns an immutable map which cannot be modified. You need to create a new map, putAll from the parameter map and remove the required key you do not want.

def reqParams = [:] << req.getParameterMap()
reqParams.remove('blah')

You have your new map as reqParams (without the undesired key value pair) and the original parameter map.