Photos where the primary subject is something other than the environment - for example, photos of people who happen to be standing in a ruined building, and where the purpose of the photograph is to present the people as the focal point, are generally not acceptable. Exceptions would include "holiday photos" where Aunt Martha is mugging for the camera in front of a picturesque ruin, or odd photos in a series which happen to include a figure. [Passers-by who happen to get into shot are an occupational hazard and exempt from any such consideration.]
Drawing a line in the sand between what is acceptable and what isn't in this area, in the concept of urban decay, is not easy; and is subject, naturally, to personal taste (and prejudice).
Similarly, there are many opinions as to what constitutes "urban decay" - I intend to develop a basic definition, in consultation with the community, when I have more time: there have been several occasions when people have commented that "[blanket subject] is not urban decay". Personally, I have yet to agree with a single one of them, but in the interest of setting boundaries - and permissions - a definition should be provided in the community rules.
The only hard-and-fast rule that I'm prepared to impose is that, if you feel inclined to post something that may be borderline in this sense, you must be prepared to defend your decision before a jury of your peers - that is, the members and audience of urban_decay.
Similarly, if you object to a posting as inappropriate, then you must be prepared to put forward a solid argument, based on objective interpretation, not personal morality or prejudice - and to defend your argument rationally.
Regardless of the rationality of your argument, pro or con, however, you must also accept that there will always be those who will continue to disagree with you.
There are often times when an image may not obviously represent urban decay - a folly in the centre of a city park may look rural, for example, but is unquestionably urban - it is therefore in your best interest to make the link with urban - or decay, for that matter - obvious to the lay person who may come across your post, but be unfamiliar with the location: inevitable, in a (literally) global community.
The rules of this community state that posters should "Give a description of the location where your photo was taken, and what it is". While this rule is not enforced, it is polite to do so, and can be in your best interest - not least because as likely as not, someone will ask anyway.
If you intend to post a link to a site that is less-obviously urban decay, I advise providing as much information as possible for potential viewers, in the context that the site may be of interest to the members here, without giving the impression that it is an urban decay site. That's just asking for trouble and may result in the post being deleted.
However, I repeat, personal invective and rudeness is not an appropriate response to any post, whether you're the accused or the accusor.