What does a repressor do in the enzymes active site?

Repressors bind to the silencers in the DNA to block the RNA polymerase from binding to the promoter of the gene to reduce gene expression, not really binding to enzymes active sites

I think what you meant was "what does an inhibitor do to the enzymes active site"?

In which case, it depends on the type of inhibitor. A competitive inhibitor has a structure similar to the substrate, hence would bind to the active site as well, competing with the substrate for the enzyme active sites, decreasing enzymatic activity. A non-competitive inhibitor binds to the allosteric site of the enzyme, causing a structural change in the enzyme active site shape. Hence the enzyme would not be able to bind to the original substrate, so enzymatic activity comes to a halt for the enzymes that are bound by the non-competitive inhibitors