I want to acknowledge a retired professor. How do I specify his name? With just Prof. or something particular?
I usually see acknowledgements in papers with just the full name of the person, regardless of their degrees or academic status. No "Prof." or other honorific would be used at all. "We would like to thank Jane Doe for helpful advice regarding our data analysis."
Where a person who previously held rank or position has now left through retiring from the position (as opposed to resignation or termination), it is not uncommon to see them listed as Rank (ret.) Person Name.
For a professor, this would be Prof. (ret.) Person Name.
In USA, a retired professor is not probably called a professor. A retired professor with doctorate can continued to be called as Dr. and identified in bio or intorduction as a former professor.