You can’t use shears to stop kelp/whines from growing. This was added to Java Edition in Snapshot 21w37a:

https://feedback.minecraft.net/hc/en-us/articles/4409293520269-Minecraft-Java-Edition-Snapshot-21w37a

• Using shears on the tip of a Cave Vine, Twisting Vine, Weeping Wine or Kelp now causes it to stop growing

Steps to Reproduce:

1. Place kelp and these vines.
2. Get shears and try to use them on the tip of the kelp and the vines.

Observed Results: 

It doesn’t work.

Expected Results:

You should be able to use shears to stop kelp/whines from growing.