It could mean two things. 1) "to put someone in a box" can mean to create a situation where they have few options or choices. To limit their possible decisions, and usually add a sense of pressure or urgency, 2) It could also mean to make assumptions about the type of person someone is, without truly knowing. Like using stereotypes, for example. Probably #1 is what you are dealing with.

To put someone in a box is to think of or judge someone in such a way that puts a limit to their potential and/or what they can achieve and do. For example, "I don't want to box myself" could mean that I don't want to think of myself as someone who is stagnant and limited to doing one thing.