there are a few mods that will do this.

the easiest mod would be "get off my lawn"

https://valheim.thunderstore.io/package/ComfyMods/GetOffMyLawn/

in the cofig, set the build piece health value to "1E+17" and anything you build with the hammer will be indestructible by normal means.

this one does the same thing, but covers more types of structures, (structures not built by the player/s)

https://www.nexusmods.com/valheim/mods/233