I have read that it is best to not allow a first year comfrey plant to flower because it will weaken the plant.I did this to my Russian 14 plants and I wondered would you also do this to the seeded variety of comfrey?I've never allowed my russian 14 plants to flower and they are doing great.
"Imagination is more valuable than knowledge".Einstein
Flowers take a lot of energy to produce, so pinching buds back helps vigor. That said, I've always let my comfrey plants flower and they've always been vigorous. Its worth an experiment. Pinch one back and not another and see how they do! I'd love to see photos if someone does this.
The first year my seed grown comfrey did not flower as it was too small. After it overwintered, it came back stong and flowered a lot. It is very vigorous and has grown large enough to chop out some root and propagate it elsewhere.
If it had flowered the first year I probably would have left it, or if it was multiple plants try pinching one and not the other like Matu said.