不许 (bù xǔ) is a direct, everyday prohibition verb meaning 'to not allow / to forbid', often used in spoken commands ('不许说话'). It is stronger and more authoritative than the milder suggestion 不要 ('don't/shouldn't'), but is less formal than the regulatory 禁止 ('to prohibit') and the written 不得 ('must not'). 不准 is a close synonym but is used less frequently.