Act immediately
As The Muse states, it would be a big mistake to wait. No bullying has ever been solved by passive waiting; on the contrary, that always only makes it worse. Act immediately, and if you manage to stem the problem in its initial phase, the better for all the people involved.
Gather evidence
Gather as much objective and verifiable information as possible. Not only will you thus establish what the situation is, but most importantly you will have material prepared for your confrontation with the bully.
Confront the bully uncompromisingly
Confront the aggressor who is the active bully. It is important you do so without compromise. It is an unpleasant topic, yet you must tackle it head on. State unambiguously to the person concerned the objective facts, why their behaviour is unacceptable and what exactly you expect from them.
If necessary, contact the HR department of company management
If you sense the situation is serious, beyond your competence, or interferes with the rights of other employees, involve the HR department or company management. Stick to objective facts, but do not underestimate the situation.
Continue to monitor the situation
Whatever the solution (a warning issued to the aggressor, calming things down, transferring the aggressor or dismissing them), you need to monitor events from then on. On your regular one-on-ones with the individuals involved, ensure the situation really has been resolved.
-mm-