In Basic Fantasy, I am a good cleric. I'm just wondering if I can still cast reverse spells. For example, I know I can cast the spell "Bless", but can I also cast "Bane"?
The Cleric's deity (or the Game Master) may prohibit certain spells.
The Basic Fantasy RPG Core Rules are available on the publisher's website. The rulebook states that Clerics pray to their deity for spells, and the deity may refuse to grant certain spells. If the spell (or reverse effect) would violate the tenets of their faith, then the Cleric probably can't prepare that spell.
On page 15, "Cleric Spells",
However, in some cases the Cleric's deity may limit the availability of certain spells; for instance, a deity devoted to healing may refuse to grant reversed healing spells.
And again on page 151, under "Acquisition of Spells",
However, note that Clerics are limited in their spell selection based on their deity, faith or ethos; for instance, a Cleric of the goddess of healing should not be surprised that his or her deity refuses to grant reversed healing spells. If a Cleric prays for a spell that is not allowed, the Game Master may choose to grant the character a different spell, or optionally (if the deity is angered) no spell at all for that “slot.”
If it's ambiguous whether your Cleric's deity would permit the spell, then the Game Master makes the final decision on whether the Cleric can use that spell.