Do you have any straight pieces of comb? If you place an empty bar between two bars of straight drawn comb the bees should draw it down straight. As they continue to build straight comb you can move the piece that you don't want further away from the nest. Once it has no brood in it you can remove the divider and then start again, probably getting rid of that divider and making a new one. If it's just cardboard that shouldn't be hard or expensive to do. From my experience, if there's a trace of comb on the cardboard divider, bees will attempt to complete it again.
While I am not a top bar enthusiast, I do use foundationless frames. We have the same issues with foundationless frames. Bees will do what bees will do. Being a beekeeper is managing those issues, working within the will of the bees, to achieve your end. Even in Langstroth hives with frames and foundation, we get burr and brace comb. It is something for you to manage. Welcome to being a beekeeper. Hope you don't mind the few stings and REALLY enjoy keeping bees.