I'll show my blocklist and explain short why I've blocked the different, aswell as "how to" on blocklist at the end. I am currently maxed combat with 18m slayer, but my blocklist have remained more or less the same since early 80's.
1: Spiritual Creatures. Horribly slow task, annoying to get to, very little cash, very little exp, aswell as fairly common.
2: Fire Giants. Longer melee only task without any noteworthy drops. Melee only tasks like this ends up well below average gp and exp rates, aswell as being very common. Not even cannon in Nieves makes it much better. Aswell as being contender for most boring task.
3: Black Demons. Unless you plan on doing your tasks on Demonic Gorillas (I'm currently bored of them, too many bosses), Black Demons is one of the worse tasks you can do, even if they're very low def. It's well below average exp and gp rates, and it being such a long task and very common, it's a higher priority block.
4: Cave Horrors. Can be ok cash compared to length on early levels, but as you're getting past lvl 80, you'll get access to many tasks that are so much better. They're fairly common and next to no exp.
5: Aberrant Spectres. Only way to kill these in an ok-ish speedy manner, is with pray, cannon and bp. That means profit goes poof, and even then the exp is sub-par. They are, however, only task weight 7 at Duradel, making them the least common on this list, and lowest prio for block of the ones I've noticed.
I'm currently keeping my 6th spot open for Kraken for when I get that pet. Getting annoyed with it, even if it might not be clearest block usually

You might also want to consider Hellhounds until you unlock cerb. They are utter shit, and extremely common (task weight 10)
The "mathematical" way of creating an efficient blocklist, is going to the wiki page of the slayer master you are using, scrolling down to the task list, and compiling a list of don't-do tasks. You then order the list by Task Weight (how common tasks are), and block the don't-do tasks that got high Task Weight, and skip the ones with low. I realize that compiling such a list is why you got this topic, but that might help you later on with some free efficiency for when having to choose between tasks to block ^^
I would also like to note that slayer WILL cover any expences you might have while slaying. So I really recommend not being "cheap" while slaying. Cannon, burst/barrage, piety etc are all great tools to get you even faster to even higher levels, where you'll earn even more cash!
Happy slaying!