If you are asked to write a reference letter, you should not write it blindly. To write a reference letter is a crucial responsibility. Thus, you are legally bound that you are telling truth about a person. Therefore, you should not write reference letters for unknown persons. You must have adequate knowledge about the candidate and his/her character. If the candidate did not work with you in the past then you cannot give a false statement of his experience or expertise.
Moreover, you should not write reference letter for those who have been against at some point in the past because this reference letter can be used to tarnish your image.
A reference letter is a factual statement; therefore, do not write anything irrelevant and do not boost the candidate too much. People who like you would definitely hire the recommended person but later on, they can argue you about your wrong statements.