modelling disutility from over consumption
Varian's Intermediate Microeconomics covers a concept called the bliss point.
If the consumed amount of a certain good is under the quantity specified by the bliss point, then the consumer would prefer to consume more of the good, all other things being equal. If the consumed amount is over the quantity specified by the bliss point, then consumer would prefer to consume less of the good, all other things being equal (i.e. it becomes a 'bad').

In case of one good, a utility function that represents this kind of preference is
$$
U(x) = -(x - x_b)^2,
$$
where $x_b$ is the bliss point quantity. For two goods, similar utility functions exist, e.g.
$$
U(x,y) = -(x - x_b)^2 - (y - y_b)^2
$$
or
$$
\hat{U}(x,y) = |x - x_b| + |y - y_b|.
$$
Both $U$ and $\hat{U}$ violates monotonicity, $U$ represents strictly convex preferences, so averages are still preferred to extremes. $\hat{U}$ represents weakly convex preferences.