I'm going to put the brakes on someone's initiative at work
Is there a reason why you want to stop them?
Their plan looks too risky to me so I need to stop them.
If it sounds too risky, you should discuss it with them and move forward with that. You shouldn't put a total stop to their work as it may reflect negatively on their record.
No one asked you, asshole.
If you continue with this attitude you'll get fired one day. If you want to be good at your job you should learn to communicate with your coworkers.