arruinaban
-they were ruining
The word arruinaban is the imperfect form of arruinar in the third person plural.
See the full arruinar conjugation

arruinar

Nunca habría llegado hasta aquí si esos tipos no lo arruinaban en el banco.
It never would've gotten to that if those guys hadn't messed up at the bank.
Bueno, hablando de nombres, quiero agradecerte el haberte asegurado de que no arruinaban el mío.
Well, on the subject of names, uh, I want to thank you for making sure that mine wasn't ruined...
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