Well you have a major quandary here my friend. Let this be a lesson in why most people do one plant per bucket, and ALWAYS do one plant per bucket if they aren't feminized seeds. There is no way to remove that plant without risking harm to the others. But if you just chop it and leave the roots they will begin to decompose and contaminate your reservoir. Hopefully someone who has dealt with a similar issue will chime in, since I'm sure this comment isn't much help or solace, but I wanted to point it out for future reference.
I did two plants in one DWC bucket once and never again. In my case it was two phenos that reacted differently to the strength of my nute solution. Same strain, but I was always either burning one or starving the other. So like I said, never again.