In StreetComplete, you can use the alternate menu (in the English version, this is the "Uh…" option at the bottom of the screen), which opens a list of options → select "There is no sign".

I fixed it manually: "fire_hydrant:diameter:signed=no" will prevent SC from asking it again.

https://github.com/streetcomplete/StreetComplete/blob/master/app/src/main/java/de/westnordost/streetcomplete/quests/fire_hydrant_diameter/AddFireHydrantDiameter.kt