To prevent a map from showing in the list, you can do two things:
1) Remove it from the server.
2) Modify /gamemodes/terrortown/info.txt as follows. Open in a text editor and find:
"fretta_maps"
{
"1" "^zm_"
"2" "^ttt_"
"3" "^dm_"
"4" "^cs_"
"5" "^de_"
}
Below that, add:
"fretta_maps_disallow"
{
"1" "^de_dust"
"2" "^de_dust2"
"3" "^dm_island"
}
And so on for any maps you want to block.
You'll want to keep a backup of the modified info.txt, gmod updates will probably overwrite it.
Showing custom titles instead of filenames for maps requires modifying Fretta, not going into that here. It seems completely useless as well, every player is used to plain filenames.