Asynchronous execution is at the heart of the Atlassian Connect Framework that ScriptRunner must use to extend Jira Cloud.
Jira fires webhooks when events and transitions occur and any user interface elements are loaded in iframes. It is not possible to veto, cancel or otherwise prevent Jira performing an action using ScriptRunner.
Scripts are written synchronously, that is, REST API calls are made in a blocking style (although async methods are provided too). For more information, you can refer to the About Jira Cloud Platform documentation which provides you with an excellent introduction to the Atlassian Connect framework.