The actions you already performed should be enough to stop milestone actions for this Service Target. However, it may be that the zSLMGen:SLM<ID>_<ServiceTargetName>_Milestone_BR`! filters were not deleted when the actions were removed.
Using Dev Studio, you could search for filters that begin zSLMGen. If you find some with the same SLMID as the Service Target involved .... and where the filter name ends with BR`!... those will be the filters that fire to perform the milestone actions for this SVT. If you do find some that were not removed for the relevant Service Target, as a temporary workaround, you could manually disable these using Dev Studio.
The Milestone filters should be deleted when you remove the related action from the Service Target and rebuild it. If that is not happening, it suggests some problem with the slmbrsvc process/activity.
Anyway, I hope some of the above helps!