Use a professional killstreak kit on a killstreak weapon

According to the TF2 Wiki...

The Strange quality was introduced alongside the Über update, available through unboxing a series 19 and onward crate. They have a kill count that shows the total amount of kills obtained with the weapon. The items will have their default name changed dependent on how many kills the weapon has. For instance, after getting ten kills the weapon will change from 'Strange' to 'Unremarkable'. Renaming a Strange weapon hides its prefix, even if it levels up after that.

It appears that it will keep the kill count, the name will simply stop updating.

This Steam forum thread could help you out a bit. Basically, what you're doing is creating a replacement for the mtp.cfg file found within the vtf files. I do believe since it's a few years old you might need to do a couple things differently. You don't need to use GCFScape at all, for one, and you just want to look at the following:

Put mtp.cfg file here:
(NOT in gcfscape, but in your steamapps folder.)

Steamapps/"USERNAME"/Team Fortress 2/tf/cfg

I can't test this right now, but I believe what you want to do instead is go inside the /tf/custom folder and create a folder named whatever you want, "Pyrovision Fix", maybe. Inside that, create a folder named "cfg" and put the mtp.cfg file there, rather than directly in /tf/cfg.

To disable pyrovision in all maps, I'm thinking that you'd just want to leave the mtp.cfg practically blank, like:

"VisionFilterShadersMapWhiteList"
{
}