First of all, you can't use this for griefing as it forces you to the closest nether portal in the overworld. Secondly, this is how nether works, and it's pretty much a vanilla mechanic. I could disable the use of nether portals in the nether but that would be extremely annoying and I don't see any reason to do it anyway. A possible solution to what you claim is however, forcing players to always go back to the overworld nether portal they used to get into the nether when entering ANY nether portal from the nether. I'll do that if I see this is being an issue, thanks for your report