As much as I like the idea of the repellent spray, I'm just too lazy to organise it . When my DS1 has had fears, I've told him that I can be a very cranky mum with scary people, and I simply won't let them stay in my house at all. I kind of role-play it, and show DS1 what/how I'd tell the bad person/monster/alien/dinosaur to leave, using my strictest, "don't mess with me I'm a very tough mum" kind of tone, and then after I've finished telling off the bad guy (and making sure he's gone), I tell DS1 that I will always protect him etc etc. So far so good, this has worked pretty well.

Hopefully you'll find that when he's over the vomitting bug he settles down again.