Unknown people and things (qualquer um, seja quem for)

Just Anyone (qualquer um)

When talking about an unspecified person, you can use the expression qualquer um to mean "anyone" or "just anyone". It is especially common in strict contexts—like VIP event security—to emphasize that a privilege or access is not for just anybody.

The VIP area is not for just anyone.
A área VIP não é para qualquer um.
We do not let just anyone enter without the permission.
Não deixamos qualquer um entrar sem a permissão.

Whoever It Is (seja quem for)

To emphasize that rules apply to absolutely anyone, no matter their identity or status, use seja quem for ("whoever it is"). This expression is built using the verb ser (to be) twice: once in the present subjunctive (seja) and once in the future subjunctive (for).

We have to block whoever it is.
Temos de barrar seja quem for.
Whoever it is, I demand the credential.
Seja quem for, exijo a credencial.

Whatever It Is (seja o que for)

When dealing with unspecified things or objects rather than people, simply swap quem (who) for o que (what). Seja o que for translates to "whatever it is" or "anything at all".

Whatever it is, we confiscate that at the door.
Seja o que for, confiscamos isso à porta.
You cannot bring whatever it is without the permission.
Não podes trazer seja o que for sem a permissão.