When trying to stop a multibranch pipeline build, I get the following error:

POST is required for org.jenkinsci.plugins.workflow.job.WorkflowRun.doStop

This error is followed by a button marked "Try POSTing" which goes to an error page (403, missing crumb).

The workaround is going to the Script Console and running:

Jenkins.instance
    .getItemByFullName("<BUILD NAME>")
    .getBranch("<BRANCH NAME>")
    .getBuildByNumber(<BUILD NUMBER>)
    .finish(hudson.model.Result.ABORTED, new java.io.IOException("Aborting build"));

But this is incredibly hacky and leaves the workflow view for the build in a state of "in progress" even though the build is no longer running.

Also, on the next restart of the Jenkins server, the build will resume running.