in the unit editor each unit has a field for tilesets (that might not be the exact name). here you can alter which tilesets this unit is available on.
also, when you are placing units on the map, you can change which tileset you are selecting units/doodads etc from, so that way you can place creeps that belong to northrend, even if you are using the outland theme for example.
if you are talking about abilities that spawn units, - they generally will just spawn the unit you set in the ability. there are abilities like "mechanical critter" where the ability spawns a random unit from the pool of units defined as critters available to that tileset. if spirit bear is like that, you can just have to change the brown bear's tileset definition to "all" , and make it so the polar bear is not available for the icecrown tileset. I assume that should solve your problem.
I have been repeating the word tilesets, I hope i am not using the wrong term. don't have time to open the editor and check currently. hope this was helpful...
