i’m not aware of a way to stop an export once you’ve kicked it off. on the export page, i can see the queued or processing status of a job that has been submitted, but i don’t see any buttons to stop the progress of such a job, though i guess you could request one to be added in the future here: https://forum.inaturalist.org/t/ideas-for-a-revamped-explore-observations-search-page/8439.
that said, even if a particular set of data to be exported was large, i would assume that iNaturalist’s servers would be powerful enough that no single export job would dramatically affect the execution of other export jobs. so i wouldn’t worry about it too much.