Hi, To take something to another level means to go above and beyond what is normal or expected. A 'joint' can be used to describe a thing generally, though it has many uses. In this case it is being used to describe a place or establishment, which is also a common usage.

Here: joint (PLACE) /dZOInt/ noun [C] 1 INFORMAL a bar or restaurant which serves cheap food and drink: We had lunch at a hamburger joint and then went to see a movie. 2 SLANG a place where people go for entertainment and which often has a bad reputation: He owned several bars in the city and ran an illegal gambling joint. We arrived at the club just before midnight and the joint was already jumping (= busy). And by saying it took it to another level the author points out that the pizzeria increased the strictness of its policy.

Banning children from a restaurant is a very extremist measure to take. It is not a simple step like putting up a sign saying "please be quiet" or "keep your children quiet", but a much stronger measure. For such a case we say "it took it to another level" or "it took it to a whole new different level", when something doesn't change in just one step or a few steps, but to extremes. By "joint" they just mean that particular pizza restaurant, so "pizza joint" = "pizza establishment".