Whether you’re faced with an offer that you’d never accept in a million years or, one may have been a really good it however you've since received a better offer, there is definitely a gracious way to say ... “Thanks but no Thanks.”
As a someone who found their self in this very position quite recently with a candidate nicely following the 'Show Your Appreciation' > 'Give a Good, Brief Reason' > 'Stay in Touch' approach, he was kind enough to send me a email directly asking for a face to face catch-up or a brief call.
I duly rang back with the candidate then explaining their rationale ... with him not shirking away from our having a straight forward conversation with the would-be employer.(me). Something I very much appreciated and someone that - given their courteous and respectful engagement - I'd have no issue staying in contact with re any future opportunities.
Although disappointing that he chose someone / somewhere else to work for, both parties parted ways wishing each other the very best. Good old fashioned conversation ... no-one hiding behind email or text ... no radio silence leaving either party in the lurch ... nice ... simple.