If you find that every call you answer is being set up as a video call, even when you're not pressing the "Answer with video" button, then it's very likely your PBX or VoIP provider is including video in the media offer for your calls.
Bria will set up a call using whatever is offered for that particular call. So for audio-only calls, the media offer should only be including audio. This needs to be corrected at the server that is handling your calls. Disabling Bria's video codecs will likely have no effect, or will cause calls to fail entirely.
If you operate your own PBX we recommend re-configuring it so that it does not offer video for audio-only calls. Asterisk servers in particular frequently have video offers for all calls enabled by default.
If you have the services of a VoIP provider we recommend reaching out to them to see if they can disable video offers for audio-only calls.