Are you talking bout the mission saves? That is because the Authors of those missions have the EnableSaving line of code enabled in their description.ext (located in the mission root folder). If you don't like the automatic saving building up MB's onto your computer I'd suggest in each mission extracting the mission from your MPMissions Root Folder and extract the .pbo mission folder. Such as in this example we will use epoch.Altis.pbo. First you need to find your MP Missions Cache Folder, which is located at: Local Disk ©: > Users > (Your PC Name) > Appdata > Local > Arma 3 > MPMissionCache, here you can find the mission you downloaded or played. Then extract it with the PBO Manager tool. Go into your description.ext of that extract .pbo and in the document file find "EnableSaving", change it to:
enableSaving false; // Saving disabled and make autosave. <---------
enableSaving true; // Saving enabled without autosave.
enableSaving [ false, false ]; // Saving disabled without autosave.
enableSaving [ false, true ]; // Saving disabled and make autosave.
enableSaving [ true, false ]; // Saving enabled without autosave.
enableSaving [ true, true ]; // Saving enabled and make autosave.
Once done save the description.ext file and then pack the pbo once again or leave it empty as epoch.Altis. (If you tend to leave it just epoch.Altis I suggest putting it in Documents > Arma 3 Other Profiles > Missions for no other further misconceptions! Hope this helps! :)