Which brings the age old question of whether it's better to provide a bad experience or sending the potentially new players home since you have not enough DMs, which is also a bad experience and just as likely to cause the potential player to not come back. Which is worse is not as clears as both proponents of either side say it is.
Speaking as a DDE coordinator, ex-convention, ex-avid living campaign player and DM... I would rather disapoint someone with not enough spots at the table but the promise of having more seats the next session than provide them with a subpar experience at the table.
I have the seen the effects of packing a table with more than 6 players in a loud, time constrained location and it is miserable for everyone.
OTOH, I usually have several folks who are playing at my DDE tables that have DMed for me in the past and will usually step up and run a last minute table for me. I trust them to run D&D Encounters sessions cold and we will get the DMs who have prepped together with them before we start to bring them up to speed.
But this is because I have actively nurtured a very deep DM pool here in Portland, OR. I have been running player and DM resources here for years. I make sure that my DMs take breaks so they don't burn out. Fact is many of my DMs (and players) go on to DM at other game stores around the city.
I know that I am lucky to have a very good game store that supports organized play of all stripes but without the sweat of people like me and my DMs Portland would not have a remotely strong program.
As for approaching a store owner about a problem DM, here is my advice. Don't just go there with the problem, go there with a solution.
If you tell a store owner that Suzy DM sucks and makes your table curdle but Suzy DM is one of the few folks that consistently shows up to DM, you are going to hit a brickwall without a substitute for her. This is doubly so if Suzy DM is a friend or long term patron.
One reason I have pull with my FLGS owner is I have consistently taken the burden of running a program off her lap and delivered. If someone is not up to snuff and not playing fair with the other players, she will back my play if I remove them from the DM slot. I have the owner's trust.
My two coppers,