A servitor and an “impostor” spirit are NOT the same thing.
A servitor is simply a blob of energy given form by a magician to do a job. They have no free will. They will not attack anyone unless that is their purpose and they are commanded to do so (many people use servitors for defense so if someone or something arrives uninvited, the servitor will attack them). If someone sent a servitor after you, you would deal with it the same way you would deal with any aggressive entity.
An egregore is basically just a servitor created by a group mind. Many occult groups create them as protectors and teachers for the use of their members.
An impostor spirit is a lower level, parasitic entity that masquerades as a stronger and more powerful being. Impostor spirits are rare, and basic spiritual hygiene such as banishing and/or cleansing, will prevent them. You can recognize them by simply asking any spirit you work with to state their true name and/or to show their seal. An impostor cannot do these things.