You can pass the list of Ad Sources to be filtered for this request via the API before calling the ad placement's Load method. During ad loading, the Ad Sources in this list will be filtered. The specific configuration is as follows:
// Filter the loading of a specific Ad Source within a placementId
[[ATAdManager sharedManager] setExludePlacementid:@"you placement id" unitIDArray:@[@"3628" ]];
// Filter the loading of ads from specific platforms within a placementId
[[ATAdManager sharedManager] setExludePlacementid:@"you placement id" networkFirmIDArray:@[@(ATNetworkFirmIDTypeFacebook),
@(ATNetworkFirmIDTypeAdmob),
@(ATNetworkFirmIDTypeCSJ)]];
Friendly Reminder:
unitID is your Ad Source ID in the Taku dashboard, as shown in the image: 
networkFirmID is defined in the ATAPI header file. Please go to the ATNetworkFirmIDType enum in ATAPI.h to check the mapping. For Swift projects, first import AnyThinkSDK, then click into import AnyThinkSDK.ATAPI to view the enum definition.
For Custom Adapter, you need to go to Dashboard -> Left Menu - Ad Platforms to check the Ad Platform ID (i.e., networkFirmID).
