When the central bank wants to encourage spending, they print money leading to inflation. Inflation encourages people to turn cash into goods and physical assets. This increases spending in the short-run and stimulates the economy.
There is no need for any entity to intervene in the economy. If people want to spend, they will spend. If they want to save then they will save.
Your "central bank" has no right to even exist. Its just a gang who think they can order people around and control them like livestock.
If a gang like that ever gains the power to do what you are suggesting, then users will abandon the currency and set up another one that isnt controlled by a gang of authoritarians.