IP Addresses are often divided into several categories. One of these is a Class C.
A Class C network block can be easily visualized by thinking of an IP address, such as 1.2.3.4. The Class C block includes all addresses under 1.2.3.X, where X is any number between 0 and 255. (That includes broadcast addresses too, which aren't necessarily relevant.)
When someone says "Lock out the entire Class C," they mean to lock out the 255 addresses in that network block. This is useful because most of the time the addresses in a Class C are associated with one another. For example, a dialup network node might have all the addresses within a class C assigned to it. Blocking that entire class C block locks out the dialup node.