I would tell him to shut up and mind his own business. I have my own decision in life and cannot be dictate by someone. If ever that he have a good reason behind that then I may or might be accepting his reasons but would not take it seriously.
You are very rude, Sometime people are sincerer with us.
Actually if someone say that to sell your bitcoins, So look what is right for you, may be his suggestion is right for us, Because bitcoin selling is depend on time, if bitcoin price down so it is good for us to sell it, and if bitcoin price is growing up than don't sell your bitcoins.
I think it greatly depends on the context. It's not excessively rude to tell someone to mind his own business, if the person in question was trying to aggressively convince a stranger to sell his Bitcoin without being asked for advice in the first place. It also depends on if the person in question gives any reasons for his/her recommendation or if he/she is just reiterating a preconceived opinion of the mainstream media without thinking by himself/herself. In the latter case, it's safe to qualify the person as misinformed and intrusive and treat him/her accordingly.
Of course it's always necessary to be open for different opinions that are based on new information and analysis. Albeit I would argue that competent people are in general not inclined to through around their insights, because they consider them precious and don't want to annoy people anyway...
ya.ya.yo!
No comment or i wouldnt tell them also 
