Here are several ways, I'll divide them if you have or not a webcam. This is purely assuming that the webcam is a mean to see the sub obeying and receiving the punishment, not a mean to ensure he/she will actually do what You say...

If you do have a webcam:

a) Make her/him kneel in front of of the computer, and pull the chair aside for a period of time. Have the sub hold his hands on his back with no restrains but "as if" his wrists were tied on his back. Tell him/her to read what You have to say and why he/she was punished and not to write back, simply to think what he/she has done to deserve your desicion.
b) Put the sub to stand in a corner, facing away from you but with cam on him/her. Tell him that you will make a buzz or a sound on his/her messenger, and he/she is NOT to turn around to the monitor until then.
c) Put her to remove an item of her clothes and place it in her purse or pocket for an entire day or a determined amount of time. Preferably intimate clothing. She will show you the underwear being removed and stashed away.
d) Tell her to get a candle, sit in front of the camera, light up the candle and hold in a hand of her or Your choice. Then have her tip the candle to a degree where the wax will slowly drip onto her other hand. Preferably not her open palm but the backside of her hand. If she is able to undress, she could drip wax onto her breasts or any other part of her body, this is up to Y/you both.
e) Have her standing naked in front of the camera, and place an object (preferably round) between her legs or knees. She is NOT to drop the object for a certain amount of time, while she DOES have to keep typing and answering to your dialogue.

If you do NOT have a camera:
Of course all the previously cited methods are good as well, but here is a list of things you can be certain she/he has done with no need to see them thru a webcam, or a digital camera shoot-and-email method.

a) Have him/her write an email to you whith the following phrase: I will obey to my Master for a x years. He/she will repeat the phrase 1000 times.
The "x" will be a number increasing from 1 to 1000 so that even if he/she uses copy and paste he can only do so much, since the number has to be changed by hand.
b) Tell him/her to go to a forum of your choice and describe the mistake she/he commited and the punishment she/he deserved by hands of His Master and ask for His forgivenesss. The topic should be answered by only his Master but others may take pleasure in her humilliation of having to write about it. You can even decide that her description should be of a minimum of 100 words or so. You will answer to her later, letting her know You have forgiven her and reminding her of her place.
c) Tell her that she is to go to work next day and at some moment during work take off her panties and get a xerox photocopy of them at some place. Doesnt have to be at her work, if she is afraid the copier is server controlled, she can go to any Kinko store. She will email you the image. The fact of having to do it is exhilarating enough even if she is not taking the clothing off her body and even if she had the item in her purse the entire day, just the fact of having to photocopy them will make her look over her shoulder before doing it.
d) You can also have her leave a mesage at a certain phone mailbox if she is not confortable enough about calling you. She should apologize, and then hang up. The call should be made at a time of Your choice, as the message will register the timestamp and You will know if she actually remembered to do it at the precise moment you ordered her to do so.
e) Find out what is the number of her nearest public phone. Tell her she is to walk there, and wait for the phone to ring. She is to pick up the phone 10 times as you call, and apologize in a low voice, then you will hang up and call again. The phrase she is to utter can be preset or agreed upon earlier. Sooner or later someobdy will walk close by and she will feel embarassed of saying whatever she is to say.