Consider the following:
One-time, one-off, one-shot, singleton, nonce.
Unique may serve, as mentioned in a comment below, and some other perhaps-less-suitable words are suggested in Antonym of "recurrent" [closed], including
isolated, seldom, infrequent, non-periodic.
The macmillandictionary.com “happening only once” entry mentions several of the words listed above, as well as just this once, unrepeatable and in/at one fell swoop. Macmillan gives two senses of the fell swoop phrase: 1, Happening or done quickly, suddenly or immediately ... at once, straight away; 2, Happening only once: once, isolated, unrepeatable. But in several other sources (1a,1b,2,3) that second sense isn't listed.
Prefixes hapl-, haplo- are “word-forming element[s] meaning "simple, single; simply, once”. As a related example, hapax legomenon means “"word occurring only once," Greek, literally "once said," from hapax "once only" + legomenon...”. Wikipedia's hapax legomenon article defines the term more precisely.