“Set its value at X”. There is no nob or “setting” to do this.

There are plenty of Forex exchanges trading RUB for any currency. Including USD, EUR and others. Governments don’t have control over all exchanges so they can’t set the price. The price is determined by supply & demand (ie: the market). Governments do have some levers they can use: buying or selling reserves to put up or downward pressure on the market price.

To limit trading of a currency you’d have to limit exchanges to any other currency. If you limit RUB-USD for example, people could still trade RUB-EUR and EUR-USD, thus still creating a defacto RUB-USD pricing.