locking uvs
 

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Not in the sense that you're talking about. The workaround is to use isolation sets. Select all the UVs and isolate them (top right icon), then display the isolation set (top left icon) and remove the UVs you want "locked" from the isolation set (bottom right icon). The removed UVs will disappear leaving only the ones in the isolation set. The bottom left will remove all the UVs from the isolation, returning the UVs to normal accessibilty.

In this image there is actually a UV to the left of the selected UV that has been removed from the isolation set, and is therefore not selectable.