How to enforce two nodes to different partition domains

I intend to use metis4.0.1 to partition an increasing mesh during simulation repeatly. I don't want to change already existing partitions but just adding the new mesh to the old ones based on the existing partitions. I noticed a forum thread that gives a way to merge the nodes into one node so that all the nodes in the merged one could be in the same domain.

My question is how can I force specified nodes into different partition domains?

Does using weights for nodes or edges help?

Thanks lot!