This is what happens in many countries when it comes to elections http://www.ireport.com/docs/DOC-270323
Even democratic countries use tricks of this type when it comes to the expats. If the current president is among the candidates and, like in Iran's case, has a target of supporters from rather conservators, nationalistic groups, it is clear that his votes are not going to come from the expats living in other countries, especially in a democracy like the US.
Then, there are a few things you can do.
One, you reduce as much as possible the centers where expats can vote. It wouldn't be excluded to find out that, for example, in the US the only voting location would be in Washington.
Two, you reduce the opening hours to, let's say, an interval quite short and inconvenient for people (during church time etc).
Three, as Ahmadinejad did, don't send enough voting forms and so on.
And just think that everything is legal and there is nothing you can do.