Please clarify: do you want to maintain the E.coli as intact cells while you inhibit nucleases within the cells? If so, I have no idea! However, iIf you are disrupting the cells, use a high concentration of EDTA (I have seen up to 50millimolar being used in some protocols). This chelates especially magnesium ions that are essential for the action of the vast majority of enzymic nucleases. IF you really do mean "chemical nuclease" you need to be more specific.