You can create a new event trigger that does not involve this field. For example, You can create a checkbox field on the opportunity that is required to be checked to create a new project. You can also add another checkbox field that signifies that the trigger has already created a project. Then you can modify the event trigger to look something like this.
1) opportunity stage = Closed_Won
2) create in clarizen = true
3) project created = false
1 & 2 & 3
If you need assistance doing this, please contact your respective account executive as ot will require PS assistance.