1/4 turn shutoff valve doesn't completely turn water off
Get a pipe cap.

This is normal with shut-off valves that are not exercised for many years.

You only really need it to slow down flow enough to change and tighten the connecting piping.  Sometimes that's all you can hope for.  That said, we've been on a campaign to replace every one in the facility, but it's almost futile - we'll have the same problem again in 30 years.