metal fill

Can we place the blockage for only Metal Fill,if its possible, Why we want to do this . Can't this be controlled using Metal Density rules ?

Yes, it's possible to block the metal filler over some design. The reason to block the metal filler is because some circuits are sensitive to noise or coupling. The metal may induce noise into circuits. In addition, the metal filler may cause matching concern if the design spec is very tight.
Metal density rule only check the density in the same metal layer. Because we may not need to block metal filler for every design, it's hard to use density rule to block metal filler