I´d like to add something here (and I´m hoping not to overstress this post 
).
Setting the phase of a service with Rest to monitored, vanished, ignored, etc. works fine for services within the group Services->Catalog of check plugins->Operating Systems->Linux-> Filesystems, Disks and RAID but not with services of other groups (at least the ones I tried).
When trying to set the phase of services from other groups i also get a response 204, which should be ok but this change is not recognised in Checkmk.
Maybe there is a setting somewhere to enable phase change via Rest for specific groups?
(I´m on 2.0.0p20)
EDIT: Sorry, it only seems to work for df and mounts (maybe all built in plugins?). I tried adding my own plugin to this group and it didn´t work.