Need to free up slow computer, choose which packs to keep installed or not.

@chickenpauper  I wouldn't bother uninstalling stuff packs or kits, if you have them.  A dozen SPs might be the same size as one expansion, and kits are smaller than SPs.  Game packs are larger than SPs but smaller than EPs, so whether it's worth uninstalling those depends on how many you have and which ones you'd like to keep.

If this is just about disabling packs without uninstalling, please see this:

https://sims4.crinrict.com/eng/2017/03/disable-single-packs/

Unless your hard drive is critically low on space, it won't do any harm to have some packs installed but not loading.  If you have at least 25 GB free after installing Sims 4, then I wouldn't bother removing any packs, and you wouldn't see any performance issues unless free storage dropped below 10-15 GB, depending on other system details.

Memory is a different story, but each pack doesn't add much to the game's overall RAM use.  The most demanding is probably Cats & Dogs, with game packs doing very little to increase memory use and stuff packs contributing almost nothing.

To answer your question, yes, you can manually delete the folders for certain packs.  Each pack has its own folder, labeled by pack type and release order, for example Get to Work is EP01 and Werewolves is GP12.  By default, they're here:

Program Files\EA Games\The Sims 4

The EA App may try to reinstall them one time, which you'll need to cancel, and you may also need to go back and delete the new folders that spawn when the App tries to reinstall those packs.  Or you could just empty out the contents of the folders but leave the folders themselves in place.

Be sure to disable automatic game updates within the EA App's settings.