OP HACK (HELP)

I had a great minecraft server playing fun with my guys and when i gone out a banned player unbanned himself and opped himslef i dont really know how and he ruined my whole server im wondering how did he did whole of this stuff i was the only op on my server...and he's name been into ops.txt

someone can tell me how did he did it im so curious

@cosminthunde
Is your server cracked?
Is it vanilla/bukkit?
Which plugins does it have?
Did someone claim to be from planetminecraft or whatever and you oped him?
Did someone ask you to test his server if it is working and you connected to it?

Btw wrong section

Bukkit server no planet minecraft and i didnt opped anyone i have a bound of plugins : BlockControl , ConfigExp , DisguiseCraft , Essentials , EssentialsChat , EssentialsGroupManager , Illegalname , LWC , NoCheat , NoWeather , Orebfuscator , PigSetspawn , PVPToogle , Regios , Vanilla , VanishNoPacket , WorldEdit , XAuth

i know is wrong section but im only disperated im out of ideas of blocking this fool
i cant even BANIP him he rejoins minuts later....im not a fool or planetminecraft bla bla bla...anyway how is they chaning my ops.txt i am the only op in ops.txt and they came there by they're own dunno how i wanna just learn how is they doing to block they ruin my work