Both are possible.
I would prefer to use “until”.
No one can enter until the security staff has arrived.
No one can enter before the security staff has arrived.
Until is more conditional. If … then … So, only if the staff is present you can enter the building. Until that moment it is forbidden to enter.
Before has a more temporal aspect. I entered the office before 6 a.m. When using “before” I would write the sentence like this: “No one can enter the building BEFORE 6 a.m., which is the moment the security staff will arrive.”