无聊 vs 闷 differences in usage
无聊 (meaningless; uninteresting; listless) 

闷 (bored; boring)

When using 无聊 to describe an event, your opinion on this event is 'it is meaningless' (therefore boring)

When using 闷 to describe an event, your opinion on this event is 'it is boring' (It just describes how you feel, no mention of the reason)

When using 无聊 to describe a person, your opinion on this  person is 'he is uninteresting' (therefore boring)

When using 闷 to describe a person, your opinion on this person is 'he is boring' (It just describes how you feel, no mention of the reason)

When using 无聊 to describe your feeling, you mean you feel listless, nothing interest you (therefore bored) 

When using 闷  to describe your feeling, your mean you feel bored (It just describes how you feel, no mention of the reason) 


(for event) 无聊 = boring (because it is meaningless in your opinion)
(for event) 闷 = boring (for whatever reason, may be the event is too long, may be things keep repeating and nothing new happen)
(for person) 无聊 = boring (because the person is uninteresting in your opinion)
(for person) 闷 = boring (for whatever reason, may be that person's topics for conversion do not interest you, maybe that person doesn't like to talk at all.)
(for feeling) 无聊 = bored (because you are not interested in anything at the moment)
(for feeling) 闷 = bored (for whatever reason, may be you are at something interesting for too long and the fun wore off, may be you are unable to do the thing that does interest you)


Example sentence:


  别再做这种无聊事了 (Stop doing meaningless things like this)


you cannot replace 无聊 with 闷 because 闷 doesn't contain the meaning of 'meaningless'


  這人很无聊，一天到晚说人事非 (this guy is very uninteresting, he just gossiping about people all day long)


you cannot replace 无聊 with 闷 because 闷 doesn't contain the meaning of 'uninteresting'


  一个人去看戏我会很闷 (I would feel bored if I go to see a movies alone )


you cannot replace 闷 with 无聊 because 无聊 contains the meaning of 'listless' but you do interested in watching movies