Refuse further contact with a foreign fellow in red pants (8)

'refuse further contact with' is the definition.
(to unfriend is to end a friendship on social media)

'a foreign fellow in red pants' is the wordplay.
'a foreign' becomes 'un' ('a' translated into French).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'in'+'red'='inred'
'inred' is an anagram of 'riend'.
'un'+'f'+'riend'='UNFRIEND'