An interface for slayer would be great addition, though, considering how people are hardly working on getting those Anger weapons, Hunter's Crossbow, I don't see argument why exactly we should block tasks permanently, while you can cancel them and get after some time, which concludes in same as task being blocked for two or more tasks in a row.
None the less, I'm agreeing with adding interface. In additional, add something like expanding slayer features, like giving larger tasks, boss tasks (that aren't on a list of slayer assignments) and maybe, adding abilities, like crafting Ferocious/Slayer rings, making Full Slayer Helmets, so not everyone could do them just by buying stuff from Grand Exchange. Now, this would also attract people to do most enjoyable skill in game
Also, one small notice. I used to make suggestion, now I saw my mistake, if you suggest something put it in details, not in 8 - 11 words. More organized and detailed suggestion is most likely to be considered as most important, while small ones, just checked and left.
In short, I agree with interface, not with blocking tasks forever.