Manual trigger is disabled if the first job cannot be triggered manually, like when you do not have the correct permission.
To disable just a particular job for everyone, for example, you must use the 'Project-based Matrix Authorization Strategy' security method in Jenkins security config. You can then override global settings on a particular job to disallow a job to be built from anyone.
(Be really careful when you do this, the 'enter' key is bound to the save action, so it is easy to save an empty config, meaning you can't access your job again)
Improvement
Minor
Manual trigger is disabled if the first job cannot be triggered manually, like when you do not have the correct permission.
To disable just a particular job for everyone, for example, you must use the 'Project-based Matrix Authorization Strategy' security method in Jenkins security config. You can then override global settings on a particular job to disallow a job to be built from anyone.
(Be really careful when you do this, the 'enter' key is bound to the save action, so it is easy to save an empty config, meaning you can't access your job again)