Well in terms of actual punishments rather than play, which is what spanking and things normally are i'm a fan of things like letters of apology. If she does something against your rules have her write you a formal email apologising, explaining why she did it and asking for your forgiveness. This forces her to take time to think about what she has done and take time to write something which you can approve.
Other good punishments are things like lines, you can do this online quite easily, watch her on cam to make sure she does all the ones set (or have her photo the lines after she has done them).
Having her stand in the corner for a period of time you set is also good.
These are all punishments that involve forcing her to think about what she has done.
A more basic punishment would be denying her something she enjoys, so if she's a bit of a pain slut and likes spankings, you refuse to give her any spankings for a set time, or non sexually you can order her not to watch tv or not to have her favourite food or treat, something like that.
For a good punishment you should always let her know what she's done wrong, what she can do in the future to be better and if possible have it so it's a punishment which'll help her to be better.