How exactly does horus the black flame dragon lv8 effect work your opponent cant activate basically any sort of spell cards?

Answer 1

They can definitely activate them, but you can negate them.

Horus Lv8 has a Quick Effect that can negate Spell Cards. If the opponent activates a Spell Card, then you can chain this negation effect to the activation. So Horus Lv8's effect will resolve and negate the Spell Card.

Unless the opponent is using a card that can't be negated (Spiritualize, for example) then yes, it will prevent him from activating any spell cards. Horus Lv8 can activate this effect in the Damage Step too, so can stop effects like Shrink if used there.

However to 'activate' a Spell Card means to place it directly on the field face-up or flip it face-up from being Set. If a Spell Card like 'Clock Tower Prison' was already active, Horus Lv8 is not allowed to chain his negation effect to the trigger that adds Clock Counters to it.