Repadmin is the "swiss army knife" of AD tools - But the following can be one of those "big red buttons" that you keep in your back pocket and hopefully never need. But sometimes it's just useful to slow things down until you figure out what's going on...
repadmin /options * +disable_outbound_repl
repadmin.exe - the tool
/options - Did you even know this was there? If not, try repadmin.exe /experthelp
* - means run against all DC's
+disable_outbound_repl - Self explanatory? I hope so...
Please don't go running commands willy-nilly in your production environment - play with repadmin.exe (or any other tool) in a lab or against some virtual DC's so you can learn what it's doing and how to use it...then sit back and let the tools do all the work for you.