Having (sadly) been on the other side of this issue, I fully plan to have guidelines to go over during first rehearsal and addressing things like that.

Obviously had you done that in the beginning you wouldn't have this problem, but hindsight is 20/20 right?

You could always frame your request in that this is your first time directing and you'd feel cheated if you didn't get the whole experience for yourself and that if you mess up you'd like it to be your own mistakes and not someone else's and if you succeed, you'd like it to be due to you and not someone else.

Yes and no.  Bear with me, this takes some explaining.

Yes, you can and should discourage & eliminate directing done by any cast member or any production team member other than yourself.
It helps to establish this by saying just that at the first meeting or read thru.  If you have a hand-out of your CT's etiquette/guidelines/rules, this can be included there.

If you are already well past the first meeting/read-thru, then I'd suggest gathering the cast & crew at the beginning of the next rehearsal and take 5 to 10 minutes to make your firm-but-friendly pronouncement on this subject.  Be clear.  Be calm.  Be specific.  Be friendly.  Be firm!

I'd say something like this:
"In a play everyone has a job they were engaged to do.
The actor's job is to act -- that is, to be responsible to craft and perform a character to the best of this ability.
It is the stage manager's job to operate the rehearsals and the show, to keep things organized and running smoothly and safely.
It is the director's job to decide everything that goes into the show, and everything that doesn't - this includes set, props, costumes, and actor performances.
You may have some ideas about the show.  You may feel the urge to share your insight with your fellow cast members ("At the end of the second scene you might trying doing this when you say that...") or with the production crew ("I could wear a different costume in Act 2" or "I don't like that prop, could I use something else?")
RESIST THE URGE!  JUST DON'T DO IT.
You are not the director.  That is not your job.
I am the director.  That is my job.
Please DO NOT direct any part of the show.
Oh yes, you may have some suggestions, probably some very good ones.  I do want to hear your idea; I may even use your idea; or I may not.
I hope you will respect the idea that what appears on stage for this show is my decision to make.
I will provide you with opportunities to make your suggestions.  I want to hear your idea to consider it.  I am open to your contribution.
But do not give any direction or notes to other cast members or production crew members unless the director hears it first.
If you have a problem with this, then perhaps you can direct the next show, but for this show I urge you to respect my role as your director by you NOT directing others in this show.
Do you have any question about what I've just said?"

Does this sound a bit pompous or control freakish or egotistical?  Perhaps it should a bit!  You've been charged with the enormous responsibility of directing a play; that job is challenging enough without interference from others.  Make yourself clear and firm on this, right from the start (or as soon as you can), and I think you'll find less frustration from "would-be directors" in your cast & production crew.

Okay that was the "yes" part of my reply.  Now for the "no" part.

As the director, part of your job MAY be to be the "total witch" from time to time, particularly with difficult actors or production crew members. 
Your the one in charge, after all. 
You are responsible to make this play look and sound like something you would enjoy attending, and your job is to get your actors and production crew to do it YOUR way. 
You don't have to be unpleasant! 
You can ask for and use suggestions from anyone in the show! 
BUT you can also reject any suggestion given, because the final product is your responsibility.


As any military person will tell you, when the platoon leader says to do something, you do it!  If it happens to be the right thing to do, so much the better! 
And, in a slightly less military vein, hopefully the platoon leader will get some ideas from his platoon, to tap their skills and expertise, so that the platoon leader can make the best possible decision for the platoon.

WADSWORTH: So, to make a long story short--
ALL GUESTS:  Too late!

I have absolutely no problem with saying to one person or the whole cast, "we're not directing by commitee, this is a monarchy and I'm queen.  If, after you try it my way, it doesn't work for you, you may speak to me privately."

You have to let them know from the beginning that while you welcome their input and creative ideas, YOU have the final say - you will take whatever they say into consideration, but YOU ultimately make the final decision.  You can do this in a nice way.  Also, if you did not do this at the beginning, it is never to late.  Sit them down and have a director-to-cast meeting, reiterating the sentiments I've mentioned.