This situation is no different from having a teenager and a toddler. The teenager (Luna) has totally different interests than the toddler (Spike). To Luna, Spike is interesting and amusing for a couple of minutes and then he becomes annoying, To Spike, everything is always interesting and amusing and and he will want more attention from Luna than she is willing to give. She is telling him, enough, leave me alone now. It is more communication than dislike or aggression. Either leave them alone and let them work it out so the kitten eventually learns when to back off, or remove the kitten to another room if he won't leave Luna alone. Restricting Luna's movement is only going to reinforce her dislike of the kitten.
luna (4) and spike is new boy (10 weeks)
I'm so sorry for the loss of your dear little guy, no matter the reason its hard to say those final goodbyes.
have some patience, the 4 y.o. isn't going to warm up to the new kitty overnight, its takes time to become friends...even with us humans, give them time, do the intro slowly...separate rooms for a few days ...introduce by allowing them to become accustomed to one anothers scent....read pip's posts for some excellent points and the utmost example of patience...
best to the new baby...do your kitties have names??