Do you have a config.txt inside of garrysmod/data/mapvote? If yes, there are allowed prefixes listed. These are mine for my TTT Server:
"MapPrefixes":{"1":"ttt_","2":"de_","3":"cs_"}
If I would like to remove de_ and cs_ maps, I'd change it to something like this:
"MapPrefixes":{"1":"ttt_"}