If a ship in the 18th century was in deep ocean and needed to stop would an anchor be useless as it may have been too deep or were there other ways of stopping?

The term from the 18th century commonly used for stopping a sailing ship is "Heaving to" or a stopped ship is "hove to." This involves setting the sails so the ship will slow down or stop because they wind blows in opposing directions on each sail.

However, "heaving to" won't stop a ship's movement if there is a current or in a storm.

You are correct that sailors don't drop a standard anchor. It will be unpredictable and may not find ground if the water is deeper than the length of the Cables the ship can use for the anchor. Sailors were careful not to drop their anchor somewhere that it was likely to be caught, "fouled", and not able to be recovered.

A sea anchor, which works by drag against the ocean can be used in a storm. If there is a current, and it's too deep for anchoring, there really isn't much that a ship can do to remain absolutely still.