I haven't used MAMEUI in awhile, but I'm pretty sure you can hide those by going to view or options maybe and selecting filters and then filtering out clones.
You could also use RomLister to get rid of clone roms entirely and save a bit of space. With a few possible exceptions, you don't really need them.
Clones are basically games that share most of the same code with their parent rom. Most of the time, they are just earlier revisions or the same game from a different region. In other words, they're usually virtually indistinguishable from the original as far as general gameplay is concerned. A few instances in which you may want the clone are:
1) The clone is the game you are more familar with (ex. Pacman is a clone of Puckman)
2) An earlier revision of the game had certain features or bugs you want that the parent does not (ex. earlier revisions of NBA Jam TE had hidden Mortal Kombat characters which are missing in later revisions)
3) The clone has special features that you want (ex. Ms. Pacman with speed up hack or Street Fighter 2 Hyper Fighting)
Other than those examples, there's really no reason to hang onto clones IMO.
Hope this helps!