Tighten up: This is the quintessential opposite of "loosen up." When you tighten up, you become more tense and self-conscious. Instead of relaxing and having fun, you become rigid and controlled.
Constrict: To constrict is to become tighter and more restricted. When you're constricted, it's difficult to move freely and naturally. This is another antonym for "loosen up."
Clamp down: This phrase conveys a sense of control and restriction. When you clamp down, you assert your authority and limit the freedom of others. It's the opposite of "loosen up" because it constricts and restricts.
Narrow: To narrow is to become smaller and more focused. When you're narrow, you're less open to new experiences and ideas.