Originally posted by Latcady:

Unfortunately you cannot stop someone using the console in their single player world, and bringing items into the multiplayer server. Aside from banning there is not much you can do.

If these friends do not care about ruining the game for others though, perhaps they're not good to play with anyway? :(

While this is correct for purely vanilla servers - anti cheat mods do exist.

For example, https://www.nexusmods.com/valheim/mods/269.

Features:
Anti-Fly/Speed
Admin Bypass
Anti-Debug Mode
Anti-Damage Boosting Hacks
Kick or Ban on Trigger

Planned Features:
Anti-Item Spawn
Chat Filters
Anti-Ghost Mode
Dedicated Server Support
Mod Blacklist