Well, the Isis only gives one mission at a time, & while getting my zoner guard rep I had the same issue you seem to be describing, as others were taking missions at the time....all I could do was sit in the bar until one appeared.
If you really want to fail the order missions to drop your rep you just have to do as I did & just be tenacious. However, dumping order missions should directly affect the order guard rep, so I'd recommend dumping 'sair missions instead, as they are easier to re-rep than Zoners.
::edit- & yes as scorch said, make sure you drop ALL other green reps a little 1st, or in your case, you will discover that your next rep will leapfrog the Order Guard & go straight to the Xenos::