How can we stop that from happening? ...... Let's discuss, because this will be interesting to see how we will be countering this unintended attack.

NONE.
There is no way for us to stop those rich people or institutional investor with what they are doing.
There is a famous quote that says "If you can't beat them, join them". What we can do right now is to just ride the waves, set a target sell price and wait after selling. Crypto market is a decentralized market where anybody can buy anything and there is no way for us to stop them. They are a group of rich people and they have a lot of money obviously. We Average Joes can't stop them from buying Bitcoin.
I know that these institutions are buying huge chunks of Bitcoin everyday and the chances of it being manipulated is going higher but there is nothing we can do with it but to be ready if in case the worst case scenario will happen.
It is funny to thing that you could stop people from buying bitcoin, that is the scary part. If there was a method where we could stop rich from buying a lot of bitcoins, how do we know that same method wouldn't be used to stop us from buying bitcoin as well? What if regulations hit us harder instead of rich people? That has been the case for regular stocks and other stuff for example.
You pay a lot more taxes for your salary than a rich guy who owns stocks that made tens of millions of dollars because his stocks went up, most of the time they pay zero until they sell, which means if my stock that worths 10 million dollars became 20 million dollars, I just made 10 million dollars and may have paid zero taxes, whereas someone who works 7.25 dollars an hour in texas during this storm without electricity and harsh working conditions just paid more taxes. I rather have no regulations in crypto like that, equality would be enough.