It seems that the time period field for reply ban doesn't validate the value type. I would expect this input to be number type, not text.
Anyway, I wrote some non-number value and could submit it, the reply ban was created.
In addition, I could submit it with the number zero, which isn't effective as well. In this case I would expect for an error.
In general, we type cast on the server side. You'll see this fairly consistently, so what you typed would be considered to be 0. As such, this is mostly expected (we could apply type=number here, though it's not necessarily a bug that it's not).
As it stands, the behavior here is consistent with all of the date period inputs. (Though yes, 0 <units> is pretty stupid in this case.)