Cave Krakens are another monster that should be avoided. They have a high Slayer task weight,meaning you're likely to encounter them frequently. But despite the frequent assignments,Cave Krakens are slow to kill. Since you can only kill them with RuneScape gold magic (preferably a Trident of the Swamp),these tasks tend to take longer than many other monsters. Additionally,their drop table is relatively poor,so you won't be earning much GP from these tasks.

The high task weight and lack of efficient XP gains make Cave Krakens prime candidates for the Block List. Blocking them will save you a lot of time in the long run,allowing you to focus on more rewarding tasks.
Black Dragons

Black Dragons might seem like a viable Slayer task,but they come with several drawbacks. These dragons have high hitpoints and require special gear to deal with,such as an Anti-Dragonfire shield or an Antifire Potion. To maximize efficiency,you also need to use the Dragon Hunter Lance,a weapon that's extremely expensive and only useful for this task.

While the Dragon Bones are good for Prayer XP,the time spent killing Black Dragons doesn't justify the rewards. Duradel may only assign you a small number of Black Dragons (10-20),which isn't ideal for maximizing XP. What you're really after are tasks with higher kill counts that can be done quickly and with efficient methods.
Greater Demons and Black Demons

Both Greater Demons and Black Demons are single-target,high-weight tasks that can be a nightmare to grind through. Duradel often assigns large quantities of these monsters (130-200),which means you'll spend a lot of time killing them with little XP return. These demons don't drop anything particularly valuable,making the task even more frustrating.

Additionally,Greater Demons and Black Demons are often found in crowded locations,where you might have to compete with other players for kills. The lack of good loot and the lengthy kill times make these monsters inefficient for training Slayer.
Spiritual Creatures

Located in the God Wars Dungeon,Spiritual Creatures are infamous for being some of the most annoying monsters in OSRS. Not only do they require special God-Armor to avoid constant attacks from other creatures in the dungeon,but their drop table is equally unremarkable.

These creatures can be incredibly tedious to RS gold for sale deal with,especially if you're trying to grind through multiple tasks. Unless you're specifically aiming for Dragon Boots,blocking Spiritual Creatures is a wise decision to save yourself the headache and inefficiency.