EV reducing berries remove 10 EV's if your Poke is at equal to, or less than 100 EVs in a stat.
Anymore EVs that you have over 100 will be automatically removed with a single berry, dropping you either down to 100, or down to 10 less than what was there before.
i.e. If a Poke has 252 EVs in a stat, and you use one EV reducing berry for that stat, the Pokes EVs in that stat will become 100.
If a Poke has 108 EVs in a stat, and you use an EV reducing berry in that stat, it's EVs will become 98.