Tactful ways of dealing with people who glob unrelated issues together in an email chain.
So I think everyone has had this.

You have an email chain talking with a group of people about Project/issue X. A lot of discussion is going on about it, then someone interjects and asks if you can quickly solve A, B, C for another project. 

Once that is done, the flood gate is open and well the email chain is now pretty much garbage because of that.

I have tried ways to nicely tell people to stop that, create a new issue & send it to support (dev's don't deal with issues without support working on theM), and let's get back on track. This doesn't work because when you play it too nicely, people think they can walk over you...

I can take the stronger route: "Please stop wasting everyones time with issues that are not pertinent to this discussion. Put that request through the proper channels and it will be dealt with. Now back on topic.", though effective, it can be construed that you are mean to those people without a spine.

This situation is too common, and well gets out of hand. Regardless of rules / boundaries people will push it, heck people push it anyways.

So what are some tactful ways of informing/enlightening people that this is not the best or more efficient way of doing this? 
Tactful ways of dealing with people who glob unrelated issues together in an email chain.
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So I think everyone has had this.

You have an email chain talking with a group of people about Project/issue X. A lot of discussion is going on about it, then someone interjects and asks if you can quickly solve A, B, C for another project. 

Once that is done, the flood gate is open and well the email chain is now pretty much garbage because of that.

I have tried ways to nicely tell people to stop that, create a new issue & send it to support (dev's don't deal with issues without support working on theM), and let's get back on track. This doesn't work because when you play it too nicely, people think they can walk over you...

I can take the stronger route: "Please stop wasting everyones time with issues that are not pertinent to this discussion. Put that request through the proper channels and it will be dealt with. Now back on topic.", though effective, it can be construed that you are mean to those people without a spine.

This situation is too common, and well gets out of hand. Regardless of rules / boundaries people will push it, heck people push it anyways.

So what are some tactful ways of informing/enlightening people that this is not the best or more efficient way of doing this? 