how would you write the sentence in english?

每次, 你一定要去做的時候, 我們都阻止不到你.
Everytime when you insisted to do so, it's unstoppable by anyone of us.
or 另一format:
No one can stop you once you insisted to do so. (once代替了每次)

We cannot stop you everytime you are determined to do something.

每次, 你一定要去做的時候, 我們都阻止不到你
When you have to do something(唔知你講緊做d咩,所以用something) every time, we are unable to stop you!
or
We can't stop you whenever you have to do something.
have to=must=一定

Hope this help!