If inhibiting S6 kinase decreases protein translation, then could inhibiting S6 kinase could possibly slow down long-term potentiation in neurons?
I can't rule it out, but it sounds a lot like trying to tune a piano with sledgehammer. 

Neuronal LTP depends on protein translation, but so does absolutely everything else in the cell. Inhibiting protein synthesis at the ribosome will block the formation of all proteins, not just the ones responsible for LTP. Unless there's a link I don't know about between LTP and total levels of protein translation, you're really going to want to look into inhibiting the production of proteins specifically responsible for LTP and not protein synthesis in general.