"No-one can stop us using the pound!"
That is true. No-one could stop us using the dollar or the yen either, if we chose to do so. It just wouldn't be smart. Neither would using the pound outside the UK currency union.
No-one can stop Panama and Ecuador using the dollar, so they do use it. But the dollar is a foreign currency, controlled by a foreign country. These two Latin American countries allow the USA to enforce fiscal and monetary discipline upon them because they can't easily do it themselves. They have to generate trade surpluses in order to accumulate domestic spending power. Their governments are not masters of their own economies.
So if Scotland wants to wrest control of monetary policy away from London only to hand it straight back again, this time with no influence over it whatsoever, then yes, no-one can stop us.
On the other hand, good luck generating the trade surplus needed to pay for the promised fairer society after the financial services industry has been forced to move south of the border in order to stay in the same jurisdiction as its lender of last resort. Most Scottish financial products are exported to the UK.
Good luck obtaining a fair share of The Bank of England's foreign currency reserves after you've refused to take a fair share of the UK national debt.
Good luck finding people to purchase Scottish government bonds when you've shown yourself likely to default whenever you don't get what you want.
But we can always console ourselves with the thought that no-one could stop us!