I think you should go under the house with candles, holy water, and a 6-pack and discuss the situation with "skull of no one knows what it is". Mario would probably freak if you asked him to accompany you, right? But if he could put on a grass skirt and maybe do a dance while you did rites, "Oh Great Skull that clogs pipes, I command you to acknowledge that you are no longer a part of earthly life and must cross over to the pasture that awaits you.." then just yell "BE GONE!" Maybe tap its head a couple times (oh, that was a redundant thing there-"tap its head", its a skull (head) already!) I crack myself up!
You might also try to command it to stop with the spiders under the house already- "Like, NOT SCARY Mr. Scull! Clogs, scary. Spiders- awww, you can do better than that! Like talk or shake the house or something!"
Already 2 rolling rocks to the wind and building up courage to drive over and do the rites myself before something awful happens! How long is the drive from Ipswich, MA? I need a map quest here.