The posts on this item or others like them would be more useful if we a) admit that regardless of the signs, the shooting may or may not proved to be justified after the facts come out and b) that people have different philosophical opinions regarding protection of property and use of deadly force which means different underlying premises will give different opinions, c) people have freedom of speech in this country and one cannot discount that these signs or ones like them may have made criminals go elsewhere for their fun and games or could actually encourage targeting of the individual posting such signs.
The signs to me can be problematic on the legal standpoint because anything that the defendant puts out for common view whether it is a sign, internet post, bartalk to others, is by definition not hearsay. But, it may or may not be admissible regardless under other provisions of the rules of criminal or civil procedure (two come to mind immediately which are prejudicial versus probative or improper character evidence. And whether or not it is admissible, the signs might be rebutted by other evidence. Depending on testimony, it could have been a gag gift from a relative or friend, etc. It could come out that the house had been repeatedly targeted in burglaries and so forth. The problem that the prosecution could have in trying to admit this evidence to show a predisposition to commit assault could also allow the defense, by opening the door, to produce evidence to disprove this. Often prosecutors have quite a bit of extraneous evidence that is never used at trial simply because it can allow the defense to muddy the waters, might get an unsympathetic ruling from a judge, and the tendency of some juries to consider if one part of the prosecution's case is doubtful then all of it should be doubted.
The civil case would largely rest on A) the criminal case outcome including facts developed in the case and B) the greater likelihood at getting evidence like signs into evidence in civil proceedings and C) whether or not homeowner's insurance can be tapped as often a settlement would ensue in such a case.
So, for the average THR poster a few takeaways. One is whether or not putting up these signs can be linked to other behavior such as threatening to shoot trespassers, irresponsible posts on social media, trash talking about killing people (like Francis the Psycho recruit in the movie Stripes), and so forth. If it is part of a pattern, then stop and reevaluate what you are doing as you are programming yourself for trouble.
Two, you can post such signs but a) it might put public pressure especially if those signs turn up in media reports, especially in more liberal areas, to have you charged and tried in a criminal case whether or not the charges are justified by the facts. As a result, some signs might involve greater expense, especially for a trial for legal fees, affect your likelihood of bail and the amount set, and submit you, your family, and loved ones to the procedural punishment that is our criminal justice system. All of this could result from a couple of signs. b) It could be a factor in holding you liable in a civil action where the standard of evidence required is lower and judges are usually looser about admitting evidence into a case than in a criminal one. Thus, civil action could result in additional punishment to you, your family, and loved ones as well as collateral consequences.
Under such stress, marriages and family relationships are often imperiled, jobs can be lost, one can be cutoff from civil society aka the Mark of Cain, and with the internet, your actions can be judged for the rest of your life by total strangers pulling up your name on an internet search.
With that risk, to me, it is not prudent to post such signs while others may feel that their freedom of expression outweighs the risks. As far as the law is concerned, a simple no trespassing/solicitation sign will suffice. Threatening violent action is overkill. One person once said that words are like swords, you want to keep them sweet and not sharp because you may have to eat your words later as events transpire.