double obsidian walls with magma sources in between would be kinda hard to get through.
Assuming no server mods, mine the outer layer of obsidian, then rapidly place a dirt block behind it to kill the magma source block there. Mine out said dirt block, and IIRC no magma should be capable of flooding out. At which point, you can mine the inner wall, add a dirt block
above your new tunnel to stop magma falling on your head, and volia, a safe tunnel. Expand from 1x1 to 2x1 and you're in! Multi-layer magma might be harder, but only a bit.
Alternately, come in through the roof. Only needs a 1x1 hole, and best of all, you're totally safe from magma while digging it! (Plus, if all you want to do is
grief the house, not steal its items, letting magma in through the ceiling is pretty much all you'd need to do. No fancy engineering or buckets required!
)
Alternately alternately, figure out how
you get in. Iron doors are cute but easily destroyed. Redstone circuits to open them, meanwhile, can be traced - if they're outside the house, you can bypass them, and if they're
inside the house, you can follow the circuitry inside. And if you have to use your own diamond pick to overcome your own defenses, then you're making your own life a lot harder in exchange for a minimal level of protection.
So, no. Unless you've got block-protection server mods, there's no way to delay a griefer for more than a few minutes. The best you can hope for is that a griefer sees your house and goes, "Nah, more fun to be had elsewhere" instead of "Wow, that house is well protected. Must be really valuable stuff inside
!"