At some point, there almost certainly will be a workshop configuration item of "accept(/reject) Manager-spawned jobs for <such'n'such>", like they already have for allowing only certain dwarves to work in them, and the newest inter-stockpile/workshop material acceptance features.
Or so I'd think. It's been a long-time suggestion/request, and with the recent additions I can see this following on, at such time as there's good reason for Toady to get stuck into it. But obviously doesn't help you in your current situation. "Blocking" workshops with other useful (or not-useful-but-suspended) jobs is the solution, as already suggested.
You could also do it without using the Manager, at all[1], and just micromanage instead (especially as it gives you better job-order control[2]). But that isn't totally in line with what you want to do, and has problems of its own when trying to discover which workshops got which jobs cancelled and need them reinstating, sometimes...
[1] Athough it is useful for queuing up more 'job items' than you have "appropriate workshops*10" slots to take (including pre-ordering items when you've not yet got the workshops at all), handily (/spammily) re-adds jobs that got cancelled for some reason and gives you a nice "you've now got your ten steel helmets!" alerts. The latter ("...alert me when this job has happened") could be a useful toggleable quality for each individual workshop job-designation. Or a job item in its own right (Messaging "...<such'n'such> workshop wishes to gain your attention, sir!"/whatever).
[2] Except in jewellery workshops where a different priority process kicks in/