。
Visitors may not remain in the hospital after 9 p.m.
Literal
Visitor [topic-は] afternoon 9-o'clock after hospital [in-に] remain [nominalizer-こと] [subject-が] can-not.
The frame ~ことができない ('cannot X') is the most neutral potential-negative in Japanese — literally 'the act of X-ing is not possible.' It's bulkier than the potential form 残れない ('cannot stay') but often preferred in formal/official language like signage or notices. 以降 ('after, from X onward') is a fixed time suffix used in formal writing.