The phrase "lets up on" means to ease off, relax or decrease pressure. The antonyms for this phrase are to increase the pressure, become more strict, or tighten the grip. These synonyms depict the opposite of taking it easy, being lenient or relaxed. To "toughen up" or be more strict could be necessary in certain circumstances, such as in the case of strict guidelines in the workplace or enforcing a certain code of conduct. Therefore, while "lets up on" is a colloquialism used to communicate less restriction, the antonyms portray the opposite, hence, should be communicated clearly when applicable.