Note – Ghost Schedulable Classes Blocking Deployment and https://success.salesforce.com/issues_view?id=a1p30000000STwPAAW may be related, but I'm not sure
Someone in our org made the poor decision to create a class called "Account," which caused errors across a few classes which used the Account standard object. Initially, I tried deleting the class, but got the error:
"This Schedulable class is in use."
The code was simply this:
public class Account {
}
In order to solve the initial issue, I changed the class name to "Accountd", which resolved the name errors, but still cannot delete the class.
Best Answer
Go Administration Setup | Monitoring | Schedule Jobs and you should see of list of currently scheduled jobs like below
Can you also go to Develop | Apex Classes and find the class you want to delete. Then click on it to show more details then click on the button Show Dependencies to identify where the class is referenced.
Hope it helps