Whether you agree or not, Somerville is not Boston, and the policy of Somerville is to disallow and remove so-called "space savers" in the street. The city just tweeted a picture of all of the space savers they have collected so far:
https://twitter.com/311Somerville/status/571307724165799937/photo/1Also, be aware that many residents simply remove space savers from the street when they come across them, even though they are just passing by and not parking in the spot.
If someone takes "your" spot, be aware, that that person most likely did not move your space saver. Be an adult and move on and find another spot.