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I am currently employed at an excellent company, and a colleague of mine from graduate school has repeatedly requested that I put his/her name forward as a recommended hire at my company.
For various reasons, this is not someone I would feel comfortable recommending for a job; even if it was with someone other than my current employer. The person is a good worker, but he/she can be very unpleasant to work with. What is the proper way to decline their request? He/she and I have many mutual colleagues in our professional network, so I would very much like to avoid creating any animosity with this person.