禁不起 is the negative potential complement of
禁 ('to bear, withstand') — note
禁 here is read jīn (1st tone), not jìn ('forbid'). Its positive counterpart
禁得起 means 'able to withstand'. Common collocations:
禁不起考验 (can't stand the test),
禁不起诱惑 (can't resist temptation).