Remember that even though Spain wasn't that great after Napolean, it was still a a force to be reckoned with. Their navy could probably pummel the Americans and their land forces could take care of the US Army. There's also the slight detail of the Spaniards being quite friendly with the British during that period.
London is likely to avoid conflicts within a few years of the end of Napolean. Spain will still be battered and likely trying to hold onto her Empire at that point - American pests trying to take unpopulated lands will likely be able to do so for cash payments or one-time border changes. California and Oregon not withstanding much of the land that became American would likely still do so in an ATL. Should a treaty be written and more encroachment occur, however, then Spain will react, perhaps during is Carlist wars.