No entity pushing

Is there any event where I can cancel the pushing of entities?

What pushing are you talking about? By pistons? Or by hitting them?

I assume by colliding.

PocketMine doesn't implement entity pushing by collision. It's only client-side.

You can prevent this by setting the DATA_NO_AI flag on entities you don't want to be pushable (is that the right word?)

Code:

$entity->setDataProperty(Entity::DATA_NO_AI, Entity::DATA_TYPE_BYTE, true);

Then to allow movement again, just set the flag to false.

(Fun fact: if you set that flag on a player it will disable the controls and stop the player moving. )

Thx

Finally I know what NO_AI means

Client-side evaluated movement.

Sounds like a better way to freeze a player

This method is mod-vulnerable. You probably need to use this as a client-side method, in addition to handling PlayerMoveEvent. You just can't trust everything the client says, or have fun with mods. And we trust everything the client says about movement, so we have players flying everywhere.