How should I respond to these students who keep on asking me not to penalize them when the penalty is deserved?

I would say that your "Stop wasting my time arguing for marks!" just needs a slightly different phrasing. So if students keep bugging you with basically the same unfounded reason to grant some exception, I would finally write something like

"I did consider your inquiry and your arguments throughly. Based on the announced rules, the present facts and the points you raised, I formed the decision that the deduction of 10% does apply in this case. My decision will not be altered after further inquiries."

I feel that it often helps to acknowledge the inquiry and state that you considered the points.

In case you are discussing with the students in person, I would go for "inquiries on grade changes have to made in written form" (paper better than email), see my answer here.

How should I respond to these students who keep on asking me not to penalize them when the penalty is deserved?

I would say that your "Stop wasting my time arguing for marks!" just needs a slightly different phrasing. So if students keep bugging you with basically the same unfounded reason to grant some exception, I would finally write something like

"I did consider your inquiry and your arguments throughly. Based on the announced rules, the present facts and the points you raised, I formed the decision that the deduction of 10% does apply in this case. My decision will not be altered after further inquiries."

I feel that it often helps to acknowledge the inquiry and state that you considered the points.

In case you are discussing with the students in person, I would go for "inquiries on grade changes have to made in written form" (paper better than email), see my answer here.

How should I respond to these students who keep on asking me not to penalize them when the penalty is deserved?

I would say that your "Stop wasting my time arguing for marks!" just needs a slightly different phrasing e.g.

"I did consider your inquiry and your arguments throughly. Based on the announced rules, the present facts and the points you raised, I formed the decision that the deduction of 10% does apply in this case. My decision will not be altered after further inquiries."

I feel that it often helps to acknowledge the inquiry and state that you considered the points.

How should I respond to these students who keep on asking me not to penalize them when the penalty is deserved?

I would say that your "Stop wasting my time arguing for marks!" just needs a slightly different phrasing. So if students keep bugging you with basically the same unfounded reason to grant some exception, I would finally write something like

"I did consider your inquiry and your arguments throughly. Based on the announced rules, the present facts and the points you raised, I formed the decision that the deduction of 10% does apply in this case. My decision will not be altered after further inquiries."

I feel that it often helps to acknowledge the inquiry and state that you considered the points.

In case you are discussing with the students in person, I would go for "inquiries on grade changes have to made in written form" (paper better than email), see my answer here.