NOTE:
A word that looks like a preposition but is actually part of a verb is called a particle.
Held up is a verb meaning “to rob.”
Therefore, up is not a preposition, and bank is not the object of a preposition.
Instead, bank is the direct object of the verb held up.
To avoid confusing prepositions with particles, test by moving the word (up) and words following it to the front of the sentence: