do not, must not, be not, mother radical (no. 80)
毋 centers on prohibition or negation: expressing that something must not happen or is not the case. The radical sense is the same visible shape used as a component in other characters, often related to 'mother' or 'do not'.
毋 is historically a pictograph of a woman with breasts, representing 'mother', but it was borrowed early on to express prohibition. The modern form is a simplified variant of 母 with the dots replaced by a single stroke.
The shape resembles a person with arms outstretched in a stopping gesture, as if saying 'do not' or 'must not'. The single vertical line through the middle reinforces the idea of a barrier or prohibition.
For ブ, imagine a stern 'boo' sound used to stop someone from doing something wrong: boo -> ブ, and the gesture says 'do not'.