。
No spoilers.
Literal
Spoilers [topic-は] prohibited [copula-です].
Stock signage / rule-notice syntax: [topic]は[prohibited-noun]です. 禁止 'prohibition' is the default noun for posted rules: compare 立入禁止 'no entry,' 駐車禁止 'no parking,' 撮影禁止 'no photography.' ネタバレ appears here as a full-fledged noun — evidence of how thoroughly normalized the former net slang has become.