How to disable a Jenkins Multibranch Pipeline project

Append /disable to the end of Job URL. Then Jenkins will redirect you to a page where you can Post. Same applies for /enable too

If you are using a recent version of the Pipeline Job plugin (I am using version 2.25 from Sep 5, 2018) and you do not see the disable option, then you can still disable the job by appending /disable to the URL of the job.

Source:

You would need to be logged in as a user who has access to write/configure builds. And if the build is Pipeline Multibranch you still won't see the disable button. If that's the case, you can append /disable to the project URL to disable it.

https://issues.jenkins-ci.org/browse/JENKINS-27299?focusedCommentId=336904&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-336904