If you're talking about a wave:
* Still: A still pond has no ripples.
* Calm: The sea was calm after the storm, no longer rippling.
If you're talking about a spreading effect:
* Halt: The rumor was halted before it could ripple through the town.
* Suppress: The government suppressed the news, preventing it from rippling out.
* Containment: The fire was contained, preventing it from rippling through the forest.
If you're talking about a small wave of sound:
* Loud: The music was loud, not a gentle ripple.
* Boom: The explosion created a boom, not a ripple.
Ultimately, the best antonym for "ripple" depends on the specific context of the sentence.