How do I fix a loose saddle? My seat on my bmx turns literally on every bump. If I tap it it'll turn and normally fall off.. The teeth on the seat look to be fairly worn.

I don't really want to spend any money as it's just a bike for riding dirt jumps. I was thinking of glueing it but is there any other way? Cheers :)
Slap some JB Weld on that stub and shove it in the post. Let it sit for one day. Smack it around a little and if it doesn't pop off you'll be good to go.