A protein that blocks the RNA polymerase from binding the DNA?

Answer: Repressor proteins

Explanation:

RNA polymerase binds to the DNA at the site called Promoter prior to beginning of transcription.

There is an operator which is found adjacent to overlapping with the promoter and functions as the binding site for a protein. This protein is called Repressor. The Repressor is a type of a gene regulatory protein-a protein that recognises specific sequence on the DNA and binds to it strongly. DNA binding proteins, such as bacterial repressors, play a prominent role in determining whether or not a particular segment of the genome will be transcribed.