Admit any amplified instruments on the understanding that they must not be plugged in. I would restrict the number of melodeons to no more tha two, otherwise the tend to co-habit and hog the session.
Spoons are fine in the right place - on the table- and as for shaky things - ok provided they dont feel they have to accompany ev erything thats played. Tho' I guess thats true of most instruments. I'm always happy to put up my banjo or guitar and listen to others playing - its gives the fingers a rest and lets others have a go.
My worst horror is the Lagerphone - especially those that have a concrete filled boot on the end as happened at on session I was at. That particular character was told in n o uncertain terms where he could put his instrument.,
Keep on playing