Unfortunately, the system does not offer a facility to busy out incoming lines.
An answering machine, set to answer selected calls advising callers to call back, may be a solution.
Busying out lines is a facility your network provider may provide but is not provided on BT Business lines.
Another option may be to migrate to BT Featureline.
The BT Featureline facilities allow lines to be set for do not disturb, so calls will not be presented to the selected lines. Other features are also available to enhance this service.
For further information see BT Featureline.