Declining well, because you may be back

Decline the offers you are not taking, promptly and in writing, through whatever system the institution uses. Two sentences is enough and no reason is required. Places get released to somebody on a waiting list, and holding one you will not use costs a stranger a year of their life.

Where someone there spent real time on you — an admissions officer who answered five emails, a professor who took a call — send a separate line saying where you are going, and thank them. Fields are smaller than they look. You may apply to that same department for a postgraduate place in four years, and the person reading it may well be the one who remembers you were decent about it.