Bharani,
I've not encountered a total block as yet but with some people, some areas don't allow the reiki to flow as smoothly. That's when I resort to crown and base together and when I return back to that spot it's generally more receptive.
Regarding clearing negative energy, do you mean in the room you are performing reiki? If so you would cleanse the room. Performing the CKR symbol on each of the walls, ceiling and floor, focusing and pushing the symbol into and repeating "I cleanse this room with CKR"
As reiki uses us for a channel you should not receive any negative energy from your client, maybe someone more informed can help you with this one, as well as the smudge stick.
Jenn