You can disable them in the Steam Manage My DLCs page in this game's entry in your Steam Library.  

Sorry I missed that. 1. Find the file "dlc_load.json", located in \Documents\Paradox Interactive\Stellaris 2. Right click it and select Edit (this can be done in any text editor) 3. To disable DLCs, add them to the "disabled_dlcs" list
For example if you want to disable Fredmans Midsummer Epistles and the Sabaton soundtrack, this is what your file should look like:

{
"disabled_dlcs":["dlc/dlc065_fredmans_midsummer_epistles/dlc065.dlc","dlc/dlc052_sabaton_soundtrack/dlc052.dlc"],
"enabled_mods":["mod/ugc_1792575623.mod"]
}