Am l right in thinking monsters can not spawn on a space with a elder sign but can move to a space with a elder sign.

Correct. If the Hound is already on the board (to use the most likely case), it can come for you, and neither a seal nor Kate's device will stop it.

Monsters can't appear at all on an Elder Sign? That's news to me. I thought the only thing elder signs did was prevent a gate from opening there. If an investigator had an encounter which called for a monster to appear, I'd have thought the monster would appear normally.

AH Manual, Page 18, Sealing Gates: "The gate is now sealed: No further gates can open and no monsters can appear at the location of the sealed gate for the remainder of the game." This is repeated at the end of the Elder Signs section as well.

It's all right Steve-O. I didn't know that rule for years.