Gosh, so many options… here’s just a few.
So yes, you definitely can spank yourself, although some people don’t like to as it feels silly. Sometimes it works better with an implement – a ruler or kitchen spoon or slice are the classic go to options.
Snaps are good too, where you stretch an elastic band between two fingers and snap them onto your skin – usually nipples or clit or labia.
For longer lasting punishment rubbing a menthol rub into your clit or arsehole can be very effective. But start out with just a little.
Then within the realm of denial, ‘no touch’ is the obvious punishment, not being allowed to even touch, let alone masturbate, for a day or two, or five.