Permanently Deleting (bad idea)
In any area in surveyor pages, you can type in the name of the asset, and the asset will jump into selection...I keep a handwritten log book of items that I use all the time (basemap A-Z & Zero-10, dighole 1x1, MB Quad line, MB Tunneltrack dark, MP Wood, UK Stone Tunnel spline...etc...
Then you can leave all the non-used assets in place where they
should be, just in case you may need them someday. If you break all the say UK assets, and you go to run a UK route someday...it will have all the content missing.

If I were say to delete an entire UK route, perhaps all it's content is also on a US route, and all the assets on the US route will come up missing, breaking the chain.
I choose to ignore all the un-used stuff...and keep a well documented handwritten log book of everything I use, and everything I change, or do.
Loco's and cars...I simply select the ones I want as "Favorites", then only a few show up in surveyor.
Also I try to keep my downloaded content to a bare minimum of absolutely needed "3rd Party" stuff, and "Built in Content"...who really needs all 27 different road names of every F-7A available on DLS and Third Party sites, when you will only use just a choice few ?