Yes - you can opt out of this - to do so, go to your settings, then click on 'ads' in the sidebar on the left and select 'edit' on the ads & friends section - there you can set "Pair my social actions with ads for" to "no one".
Are you sure that's what that's for? When I go to that setting (which I'd already set to "no one" in an earlier privacy audit), the example it shows of what it controls is very different from the "related post" issue that's being complained about here.