。
No entry.
Literal
Trespassing [object-を] prohibit.
立ち入りを禁ず is a classical/formal prohibition. 禁ず (きんず) is the classical form of 禁じる (to prohibit). 立ち入り (entering/trespassing) is the noun form of 立ち入る. This is the language of signs and official notices.