When a rogue brown dwarf star hits a white dwarf star, will the collision at most cause a nova?
Or can the explosion be heavier?
Collisions between a rogue brown dwarf and any other star would be very rare because the space between them is so vast. I don't want to say it'd never happen, but it would be a rare event. It's much more common for two stars that are already in the same system to collide by spiraling into each other, usually by tidal decay.
A collision with a brown dwarf and a white dwarf has been recognized. The nova of 1670 didn't behave like an exploding star and recent research identified it as spiraling in collision between a brown dwarf and a white dwarf. Original article and abstract here.
As for what kind of explosion is likely. I would imagine something like a type-A supernova would be possible if there was sufficient mass to exceed the Chandrasekhar limit of about 1.4 solar masses. Because brown dwarfs are much less mass than that, about .013 to about .08 solar masses, the white dwarf would need to be at least 1.32 solar masses prior to the collision. Most white dwarfs are less than that.
CK Vulpeculae, the 1670 nova wasn't a type 1a supernova. If it had been, it would have been over 100 times brighter than Venus (ballpark estimate) based on it's distance 2280 light years (plus/minus 490). It would have been many times brighter than it's visible but not very bright peak of 2.6. SN 1604, for comparison was about 20,000 light years distant and it was brighter than any star in the sky and visible during the day at it's peak.
The mass of the two colliding objects and energy of collision would have considerable effect on the brightness, but, unless the Chandrasekar limit is reached, and keeping in mind that direct collisions would be rare, and it wouldn't hit all at once, the result should be much less energetic than a supernova, but if it's close enough, it could still be a visible nova in the sky, like CK Vulpeculae in 1670.