This novel, by Diana Wynne Jones, was published in 1984, so you certainly could have read it in the late 90s or early 2000s. The protagonist is a young boy.
The book features seven siblings, who could accurately be described as wizards, each of whom “farms” an aspect of a small town’s infrastructure.
Quentin, at that point, was yelling, "Face the facts, you stupid
woman! This town is run by seven megalomaniac wizards!" He blinked at
the sudden light and rounded on the Goon. "You!" he bawled. "I hope
you’re taking this down in shorthand. I want Archer to know!"
One of these wizards is giant, the titular Goon (shown on the front of the book).
In the end the wizards are sent away in a spaceship:
Quentin typed a loud full stop and stood up. They all crowded to the
window and looked into the yard, where the caretaker still stood,
looking suspiciously at the strange moonlit building inside the
scaffolding. A wide smile of light appeared in the domed roof, to the
right of the great head of Venturus. The smile widened like a moon, to
half, then three-quarters, and then to a blaze that struck upward into
the blue clouds of the night. There was a gigantic mutter of power, so
enormous that the window rattled and everything in the room shook. The
girders, and the temple with its opened dome, blurred with it. Their
ears went dead. Then, slowly rising from the opened dome, came the
spaceship, silvery and stately, up and up, straight as a pencil, into
the light of the dome, up out of that light, into the light of the
moon, faster and faster and faster. Their eyes followed it up, then
up, until its tail spurted white light. A blunt cough of energy came
to their dead ears. Against the moon they saw the ship slant sideways
and become a shooting star for a second, up and out.