The antonyms for the phrase "letting flow" include restricting, controlling, suppressing, holding back, and blocking. When we restrict something, we prevent it from freely moving or developing. Likewise, controlling refers to the act of directing or regulating something. Suppression means to stifle or subdue something, while holding back means to restrain or impede something. Blocking, on the other hand, involves stopping something from moving forward or preventing its progress. While letting flow implies an effortless and natural movement, these antonyms indicate a process of restraint, hindrance, and limitation.