These trained bamboo plants are tricky, because they require that you continually train them to a post when growth is going to keep that shape. When allowed to grow normally, of course they grow straight, and fast. You can trim that top growth off without harming the overall plant (use the 1/3 rule, of course), but you'll likely get multiple offshoots where you do the cutting because the trunk of bamboo is fibrous, and will regenerate new leaves from cuts, that will eventually turn into shoots over time. You might even get some new shoots from the soil line after spurring new growth after cutting, which is a positive or negative depending on how you see it.

If you wish to keep it small, just set a reminder to trim it every so often and keep the new growth to where you'd like it. Treat it like a Bonsai.