One person false reporting will get a project taken down
It takes several reports for a project to be automatically unshared. Additionally, there is a reputation system in place so users who have made false reports in the past will not be able to automatically get a project taken down.
Mass reporting will get a user permanently banned
While a project will be automatically unshared and the creator temporarily banned if it is reported by several users, the project will be reshared and the creator unbanned after the Scratch Team reviews the reports if they find there was nothing wrong with it. Additionally, the automatic ban is short (less than one day).
There is nothing stopping users from false/mass reporting projects
Someone was banned because they were false reported
While users can be temporarily banned for a project receiving a lot of reports, the bans are short lived. After the Scratch Team reviews the project, if they find nothing wrong with it they put it back up. However, users who have been banned for other reasons have been known to claim they were false reported to gain sympathy and to avoid admitting what they actually did.
What To Do About It
Report any instances of users threatening to false/mass report and do not respond. Other than that, there is not much that can be done since false reporting is already against the Community Guidelines and will result in consequences for those who do it or threaten to do it.