It does not work because you are missing logic in your writes.
When you push a button, no matter the buttons state, you will always do the same digital writes. You need an if statement that takes care of your button release that stops everything.
Push mode works like this, when you push and hold a button, BLYNK_WRITE is called and executed once, when you release said button it is called again.
First you need a function that stops your motors.
Then edit your writes to this, I'll just take one for an example:
if (param.asInt() == 1)
Now edit the rest to achieve your wish