Does anyone follow directions anymore?
I guess it really depends on how bad the deviation is. For example, if you're doing a 5e game, and you say the PHB is the only source material, I don't think it's unacceptable for someone to suggest the Elemental Evil Player's Companion since it's a free download. However, if they wanted to use something out of the Sword Coast Adventure's Guide, that's a different story since that is obviously something the DM either doesn't have or doesn't want to use.
So, I'm not sure how well I can comment here since I feel like I'm missing a lot of detail, so I'll give two thoughts.
If someone is suggesting something that comes from freely gained source material (free download, SRD site, etc), I don't think it's an unreasonable request. Sure, the rules have been stated, however, it wouldn't be going too far out of anyone's way to at least consider. More than likely, it's just something simple that they either really enjoy, or really want to try.
On the flip side, there are those special snowflakes, that even though you said Human only campaign, still try to submit their uber-homebrewed Half-Dragon Weretiger race and special class that they PROMISE is balanced (even though they wouldn't know balance if a set of scales *cough*-smacked them across the face).
At the end of the day, fun is fun. By that same token, respect a DM's rules.