How prevalent are spoofers in your area?

Depends on the gym for me.
Most of the busy gyms have a limited number of spoofers if any. Can largely identify real players in the gyms. Although it's possible some people spoof to fill up the gym after someone physically there takes it.

The less busy areas (e.g. the gym closest to my house) are absolutely full of spoofers. Aside from me on red, and one friend on yellow, there may be 3-5 other legitimate players that occupy that gym. Whenever I take it as red, it fills up quite quickly with at least 3 spoofers. When my friend takes it as yellow it often takes a long time to fill up, so possibly no yellow spoofers. Blue has at least 6 spoofers if not more.

I don't think the nerf was to their advantage though. A demotivated Blissey was easy anyways, and why would a spoofer target a fully powered gym when he can just spoof to a weaker one? It's only physical presence requirement that makes big gyms a threat - as I can't easily just pick a different gym to target.

I honestly think it just depends on where you live.

I'm in the Sacramento area and im relatively convinced I've never been kicked out of a gym by nor had to kick a spoofer out of a gym. There just aren't any good reasons or spoofers to chase after Sac's gyms, as most of them have 4 hour or shorter turn over.
I'd imagine areas that either have a ton of valuable and semi-rare pokemon spawns (such as the boardwalk in Santa Cruz) or more rural areas that have low turnover would experience a much larger spoofer population... but I have no evidence to back that up. Just my experience playing pogo and extrapolating on which gyms would be most likely to attract the attention of spoofers.

Edit: Actually I take that back. There is one park which is a complete dead zone near my house which spoofers like to fight over, since physical presence at that gym is literally impossible otherwise, but most of those spoofers are locals.

my entire area is full of spoofers/ multiaccounters

and niantic has banned exactly 0

Just after they relaxed the restrictions on spoofers, I went to take down a demotivated Blue gym (I'm yellow). The pokemon in the gym were owned by:

Henry12
Henry14
Henry11
Henry09
Henry10
Henry08

Not sure where his pal Henry13 went, but this was clearly a multi-accounting spoofer. Never saw Henry## at any other gym in my neighborhood, or that one, ever again.

All in all, though, it wasn't a big deal. Henry## wasn't berrying the gym, and if he got paid because I took him down, well, fine.

We dont really have that here anymore. Last one seemed to have lost interest a while ago

However there seem to be plenty accounts who fill up gyms. Since there are few yellow players here its a little suspicious that gyms are full mostly with names I never heard about. I dont really mind it though as long as they not take down gyms or Grb everything its ok in my book

I live in a small town in SW England, and there are several known players who persist in wrecking all of the gyms in the area from time-to-time. Some of these players are known to the community, whereas others we are yet to meet and as a result are unable to each out to prevent them from doing it. Most of the time we seem to be pretty safe though (as our community doesn't tolerate spoofing), whereas the next town over encourages it in terms of attending raids during work hours.