It all depends on the method for evolution.
For example, say your Bulbasaur reaches L.16 and begins to evolve into an Ivysaur. You can STOP the evolution at this point, BUT once your Bulbasaur reaches L.17, it will try to evolve again.
HOWEVER, since Eevee's evolution into Umbreon/Espeon is based on its happiness, you could THEORETICALLY (I haven't tested it yet) get it to not want to evolve the next Level it reaches if you make its happiness go down enough that it wouldn't evolve normally. Again, I haven't tested this, but it would be a good thing to check.