。
Dirty jokes are off-limits~
Literal
Bottom-topic [topic-は] prohibition is-[yo~].
しもネタ is a compound: 下 (しも, 'lower, bottom') + ネタ ('topic, material'). Refers to sexual or crude jokes — 'below-the-belt humor.' カタカナ ネタ is the casual spelling of the noun meaning 'joke material, content, topic.' 禁止 ('prohibition, forbidden') is a Sino-Japanese compound. よ with extended ~ gives a drawn-out, playful admonishment. The whole utterance is a soft warning.