Una vez ustedes cancelaron el evento.

It is the first option because the second option does not exist. The first option is conjugated in the preterite form (preterite perfect simple) of the indicative.
Past perfect simple of the indicative of the verb "cancelar" (cancel):
- Yo cancelé
- Tú cancelaste
- Usted canceló
- Él canceló
- Ella canceló
- Nosotros/as cancelamos
- Vosotros/as cancelasteis
- Ustedes cancelaron
- Ellos/as cancelaron
MissSpanish