I've suggested this in the past. Nothing came of it...
I wouldn't say nothing came of it. It just wasn't implemented. I know most if not all of the mods on up read this forum. A bunch of good ideas come through here. Some are used, some are not, some are refined and operate in a way different than originally suggested.
It's also good to have these things brought back up once in a while. What wasn't an idea we would've done last year might be one we'd do this year. Also as new people come to the forum and older members have traversed other forums, they might have a new idea for implementing it.
In the case of signature spam we have to think, "Well what would spammers do instead?", "What is the impact to normal users?", and "How often do we have to cleanup this mess and how troublesome is it?". We've even read through documentation on spam bots that you can purchase to see what the software's reaction typically is to this.
Overall I think it was found that the spammers would adjust (so little to no impact to them), new users might get frustrated, and if they didn't put spam links in their signatures, they'd put them in their posts. I actually get more frustrated cleaning up people that quote spam than I do cleaning up the actual spam although it's fairly rare.
I know sushi's post a bit up there quoting Blue Velvet's fake spam was in jest, but if Blue Velvet had been a spammer and then sushi quoted it 2 pages later in the thread saying something like "OMG Spam!", the odds of us finding the quote of it is fairly slim. Now if BV had had the link in the signature area, sushi wouldn't have quoted it and while the quote might remain, the link would not.
PS. Please don't read this spammers.
PSS. Please don't quote spam.