dispel magic doesnt work on an eidolon.
Banishment, dismissal and a few other methods works. One of the most common, is your summoner to fall asleep (magical or normal), fall unconscious or getting killed.
ps: your eidolon actually goes back to its plane every time your summoner goes to sleep after a long hard day.
ps2: protection spells like protection from law/evil/good etc. doesnt work on Eidolons and can attack normally.