"I am completely against it."
Translation:Ik ben er volstrekt tegen.
'Het' can be used as an indefinite or personal pronoun when it's the subject of a sentence, for instance 'Het is eng' (It is creepy).
Same goes for when it's the object: 'Ik vind het leuk' (I like it).
To my knowledge 'het' as an indefinite or personal pronoun can never be used with a preposition. The sentence "ik ben volstrekt tegen het" sounds like 'het' is used as an article and the noun behind it is missing.