There are two major devices for achieving negative feedforward or feedback. One way is to prevent the relevant enzymes from being able to perform-that is, to "clog up" their active sites. This is called inhibition. The other way is to prevent the relevant enzymes from ever being manufactured! This is called repression. Conceptually, inhibition is simple: you just block up the active site of the first enzyme in the assembly line, and the whole process of synthesis gets stopped dead.
What about positive feedback and feedforward? Here again, there are two options: (1) unclog the clogged enzymes (un-inhibit), or (2) stop the repression of the relevant operon. (Notice how nature seems to love double-negations! Probably there is some very deep reason for this.)
See Gene expression regulation too