Individual rates can have different minimum booking periods which can cause the actual minimum booking period on any day to be higher but not lower than this value.
There is a hierarchy in Beds24.com for which value is used :-
If you set the Minimum Stay here this can not be lowered with any other setting, (Rates/Daily Prices/Calendar)
Set you base Minimum Stay here and increase your minimum stay, say for a special weekend/period, in (Rates/Daily Prices/Calendar)
The hierarchy order :-
1) Room Setup
2) Rates/Daily Prices
3) Calendar