It must be a middle, or even deponent, i.e. a medio-passive in form, but active in meaning, for actually its active form βιάζω is very seldom used and by and large substituted with βιάζομαι with the same active meaning. 

Actually, here it must be active, because it is we who inflict violence on our fallen nature and not succumb to its urges, but fight against it “a good fight” (2 Tim. 4:7-8) through God’s aiding grace in us, with which we freely co-act - for we are called to be co-actors of God /1 Cor. 3:9/ who powerfully acts in us /Col. 1:29/ - or, to express the same otherwise, jointly with Christ fight against our sinful passions, becoming their victors and masters, rather than being defeated and enslaved by them (2 Peter 2:19). 

This is what “to power through”, or “to inflict violence” means, for it is our sins that block Holy Spirit’s presence in us, and when we defeat through grace of God our sins, then Holy Spirit reigns in us and we are truly free (2 Cor. 3:17), which freedom is called also a citizenship of the Heavenly Kingdom. 

Yet, it can have a meaning kneaded into it that we also undergo a salvific action of grace, but only if we also co-act with it. To give an analogy: Glen Guld undergoes a pleasure of performing Bach’s music on piano, so as that he even audibly moans with a pleasure, but this undergoing is happening with him only if he co-acts with the energy of Bach’s invisible and unheard music that potentially and causally precedes him playing it and enjoying it. Such verb can be called an “active-passive” one, even if no such category is there in Greek grammar books; but who cares about grammar books when a spiritual reality is to be conveyed or explained?! For the entrance into the Kingdom is not through letters and grammar, but power of God assumed and put to practice in vanquishing our sinfulness: *Quia non cognovi literaturam, introibo in potentias Domini* (Psalm 71:15).