So I'm pretty such alot of other people has had this problem too, you install this addon, you have a little fun with it, and maybe you get bored of it, so you uninstall it. Except, it's still acting as if you never uninstalled it. [Acting kind of like a tick where you can't get rid of it easily, hence the name.]

I first encountered this when I loaded up a map before.

[Just a note: I did NOT have this addon previous to downloading it and it still showed up as if I did own it. so I assume I got it via server downloads or something else.]

Anyways, I loaded a map and I came across the class change menu. I did not know what Homestuck was at the time and I certainly don't remember downloading this. So I fiddle around with the character creator screen until it froze my computer. [Yes I owned a kinda old computer.]
Next thing I can remember with it was I think when I was playing on the map gm_construct and I had now downloaded the addon after encountering it again. It was decent, but once more, I knew NOTHING about Homestuck, so I was clueless. Anyways, I get slightly bored with it or I thought that it may ruin the roleplays I do with my friends sometimes on Gmod and I delete it. So it's gone away now right? WRONG.

Now before we go any further, I would like to mention to any die hard or just in general fans of this addon that I am NOT hating on the addon. I am just stating what happened to me and asking if there's a fix for it.

Anyways, after I learnt how to gain more storage, [Aka deleting server addons] and after I loaded up a map once more I saw the text "Downloading Homestuck Playset 1". And I thought "What the heck?". So I cancel, and I restart gmod, same thing. So I waited what could have been an hour or so doing things like eating cereal. And once I loaded in, it popped up. The class change screen. I try to change the map manually but it's no use. And I toggle it so that it doesn't show up on spawn. And then I check the Entities menu to make sure it's not in effect. No big suprise, IT IS. So I restart Gmod right? and I'm kinda fine, no HP1 addon right? well...after I changed my map manually, [To go from Singleplayer to Multiplayer.] it shows up! and for some reason so does Mad Cows FOF Weapons. [But now I have it installed cause I liked it so not a big deal.] But yeah! and before you say something about deleting the .gma file for it, here are some things I would like to mention:

#1: I couldn't find the .gma file and I couldn't remember the command to remove the .gmas for unused addons. [BTW last time I checked, I'm pretty sure it had a couple of addons which I DID use and were NOT disabled or uninstalled.]

#2: I am NOT going to delete my entire garrysmod folder and just verify game cache. Cause I'm pretty sure that would screw up my Gmod in some way. Even with the game verified. [BTW I had a similer experience with the addon Gas Cans, which it I guess ovewritten my old fires and now I can't see them. I know the creator said it was a problem with my PC in the description, but I already tried everything except for updating my graphics card and I am NOT going to spend money on an expensive graphics card. I would rather buy an upgraded PC.]

So if you have a fix for this: post it here or on my profile. Thank you for reading and have a great day!