There's some guidance in the link below - scroll down to Wall Trained Shrubs and click on the embedded link there. I know you're growing it against a fence,but the training/pruning will be similar

https://www.rhs.org.uk/advice/profile?PID=431

You can certainly cut off any growth on the fence side. The usual way of training is using wires but I have managed without, using garden canes to bend forward-facing branches sideways when they get long enough. Its not pretty but it works. They are amenable and forgiving shrubs so don't be nervous.
If your fence has wooden uprights you can buy eyes for the wires that will screw into the posts. How close to the fence have you planted them?

I grow mine as a hedge which has to stay at a certain height so we can see to get out of the gate! This involves hand pruning at least 40 ft hedge so I don't cut off either the flowering buds or the berries! Not ideal, but looks stunning in flower and with the berries.
It's as tough as they come so just hack it off where you don't want it, you will have to do something amazingly drastic to kill it off. When it gets bigger the birds love it for nesting.

Thats a great question! I have a young one that’s currently in a pot and looks loke it only wants to grow out, and not up! I shall leave it until it decides its time to go vertical before removing any of the low growing horizontal branches.