[SalesForce] Trigger Validation on QuoteLineItem Based on Quote Status

I've made a research, try a few things…but I don't really see a good example where to understand how to do this.

I want an error messaget to be shown when I try to update or delete a QuoteLineItem from the Related List within the Quote detail page.

In a "before insert" and "before delete" trigger for the QuoteLineItem object I would like to access to the Status field from the Quote parent object and if the Status is "Approved" interrupt the insert/delete process.

I've come to this piece of code:

    trigger blockDelete on QuoteLineItem(before delete, before update) {

    QuoteLineItem myVar = [SELECT Quote.Id, Quote.Status from QuoteLineItem where Id IN: trigger.old];

    if (Trigger.isUpdate){
        for (QuoteLineItem q : Trigger.old){
                if(myVar.Quote.Status == 'Aprobado') {
                    // Adding the error to the new object
                    Trigger.newMap.get(q.Id).addError('Quote approved  --> QuoteLineItem can not be modified');
                }
            } 
    } else {
        for (QuoteLineItem q : Trigger.old){
                if(myVar.Quote.Status == 'Aprobado') {
                    // Adding the error to the new object
                    myVar.Quote.Id.addError('Quote approved  --> QuoteLineItem can not be deleted');
                }
            } 
    }

}

But I'm still not able to make it work completely…

UPDATE

I've accomplish to avoid the edition of a QuoteLineItem by using a Validation Rule like suggested in on of the answers:

             ISPICKVAL(Quote.Status, 'Approved')

But this isn't working when it comes to avoid deletion.

So, I've also tried with a trigger (I've updated the code based on other answer) and it also works fine for avoiding the edition but for the deletion doesn't work neither. It redirects me to a error page:

There were custom validation error(s) encountered while saving the
affected record(s). The first validation error encountered was "Apex
trigger blockDelete caused an unexpected exception, contact your
administrator: blockDelete: execution of BeforeDelete caused by:
System.FinalException: SObject row does not allow errors:
Trigger.blockDelete: line 16, column 1".