As pointed out, both are used more or less synonymously, so never  trust that the user ment the same thing you do.

Additional to the "you physically can´t / you are nto allowed to" meaning, there are some additional hints:

"Kein Zutritt" is mostly used for closed spaces (i.e. buildings) while "Kein Zugang" could also describe an open place, e.g. a garden or park.

While "Kein Zutritt" always means "you are not allowed to enter" and describes the *object* you might wish to enter, "kein Zugang" could describe the *access path* or way ahead of you. It could mean "this way will not lead to the building, please  use the next one further left.