There's no specific way to disable the speed control. It's part of the Now Playing screen. The only way to remove this is to stop the Now Playing screen from being automatically displayed in the first place.
"The Now Playing app on the Apple Watch offers quick access to controls for your music and other audio on your Apple devices, particularly your iPhone. But by default, Now Playing pops up automatically, and that can get really annoying, especially if you're trying to keep an eye on a timer and your Apple Watch keeps switching over to Now Playing."
How to stop Now Playing from launching on your Apple Watch
- Press the Digital Crown on your Apple Watch.
Settings on the list of apps or in the grid view.
- Tap the switch next to
Auto-Launch Audio Apps so it's in the gray "off" position.