There’s some structure that we might extrapolate from the rules we know. Normally, heroes cast spells. Any hero grants you the ability to cast all the spells of its spec, any max level hero grants you the ability to cast its ultimate if it was max level at the start of the turn, and any hero grants you the ability to cast minor spells, but some of them will cost extra. When you have multiple ways to cast a spell, you are sort of forced by the game to use the best one. That is, if you control Orpal and River, you cannot choose to pay 3 to cast Sacrifice the Weak. To add some interpretation to this, we could say you have one StW-capability with a +1 cost penalty, and 1 StW-capability without the penalty, and the game makes you use the better one.
Lost Temple reads, in part, “Players don’t need heroes to cast spells.” A reasonable interpretation rules wise would be that the map card lets everyone cast every spell (with no cost penalty). So if you have Orpal and River and Lost Temple, you now have 3 StW-capabilities. A card that says “Your heroes can’t cast spells” should only get rid of 2 of those, the ones that actually come from heroes (the ones which you can safely ignore anyway when playing on Lost Temple).
So to me it seems like you can use Free Speech on Lost Temple to remove abilities from your opponent’s heroes, but your opponent will still be able to cast whatever spells they like. Also, it seems like Lost Temple is a very silly map.
@Sirlin what do you think?