In general ‘delete_others_topics’ should allow to delete topic created by other user. User without this capability should be able to delete own topics only.
Do you use a role provided by bbPress, like participant? Or do you grant bbPress capabilities to a custom role?
Check full list of capabilities available to your user. Go to Users and click ‘Capabilities’ link under the user row. Click ‘Granted Only’ checkbox, make screenshot and send a link to it here or screenshot itself to support [at-sign] role-editor.com.