Yes, we can "deleted" anything including campaign, ad group, ads, keyword... We can even "deleted" or "deactivate" the adwords account itself.
However, all the data (keyword, ads, campaign, adwords account) that we created will not be "removed from Google server/database).
Once you created a campaign, it will always remain in your adwords account even after you delete it. The only changes is status of the campaign change from "active" to "deleted".
If you decide to stop using adwords, you can delete all your campaign, ads, keywords and deactivate your adwords account. However, whenever you want to re-use adwords, you can easily re-activate the adwords account by contact adwords support team. With a few mouse click after entering your valid billing info, you can bring back all your campaign, keywords & ads to live.
In addition, every changes your make (such as bid price, rename ads, add & remove keywords) , you can retrieve it from "Account History" <- not sure about this name.
Which means, whatever we create in our adwords account, Google will store it permanently. You can't remove all the data (campaign, keywords, ads) that is being created. You only can change it into the status "delete".
For example:
When we "delete" a file in Windows, it will be move to "Recycle Bin". We can restore the file by dragging it out from "Recycle Bin" as long as we do not "Empty Recycle Bin".
In Google AdWords, we only can move it to "Recycle Bin", we do not have the option to "Empty Recycle Bin".
Do we have the option to permanently remove our adwords data (campaign, adgroup, ads, keyword & other setting) from Google AdWords Server ?