If your attack is successful, your target is pushed back 5 feet. For every 5 by which your attack exceeds your opponent’s CMD you can push the target back an additional 5 feet.
There's no provision for not pushing them, any more than there is a rule for not doing any damage when you hit an enemy with a sword.
The rules assume you want to knock them back. Assuming this is not PFS, I would ask the GM can you avoid knocking them back.
If my Bull Rush attempt was succesful, can I choose not to push opponent from his square at all? (having Merciless Rush feat and Squash Flat feat I am totally satisfied with damage and trip attempt from Bull Rush and do not want move at all)
RAW, pushing for the first 5ft is mandatory, the rest is optional.
Squash Flat assumed you moved them st least 5ft, since the bullrush needs to exceed by 5 or more. There is nothing optional about the movement of bullush RAW. If you push them somehow 100ft, they go flying 100ft.
You can move with them if you have movement left.
Well it does say, "For every 5 by which your attack exceeds your opponent’s CMD you can push the target back an additional 5 feet." So as Yorien says, it seems like all the movement after the first 5 feet is optional.
Which is reasonable. The world's greatest warrior should have some control over how far they shove someone, just as the world's greatest acrobat should be able to jump ten feet rather than thirty feet if that's what they want to do.