If you wanted to prevent a regulatory protein from changing gene expression, you would have to prevent physical contact between the protein and .

DNA

Explanation:

Regulatory proteins can prevent and activate gene expression based on their binding with DNA sequences under specific conditions. If the ability of regulatory protein to bind with DNA sequence is altered, they do not express genes. For example, Lac I is a regulatory protein which when binds to DNA and prevents gene expression. This happens because of absence of signal molecule and when this signal molecule is present, the Lac I bind with it and releases thereby expressing the gene.