modelling disutility from over consumption In introductory economics courses the concept of marginal utility is illustrated through simple examples like how much benefit one gets from eating another slice of pizza (i.e first slice provides 100 utils, second slice 50 etc). 

I was wondering if there exists a utility function that can allow for the possibility of over consumption (i.e. a function that would produce -50 utils from the 10th slice of pizza ).

I know there would be issues with this since utility functions are only defined up to monotonic transformations, and concavity in our utility functions would violate the "averages preferred to extremes" assumption. 

Does such a function exist?
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.