Every competent individual has a right to refuse treatment or to state circumstances in which they would not wish treatment to be offered to them.
In the event that this does not include potentially life sustaining treatment, this can be successfully discussed with your GP or other medical practitioner and formally recorded in your notes. This will be sufficient to record your objection to the proposed treatment should the need arise. As it is not life sustaining, it is likely that the practitioner will be able to find an alternative treatment that you do not object to.
It is also recognised that such wishes can be formally recorded in an Advanced Directive, where your refusal of certain specific treatments or circumstances in which you would not want treatment to be offered are set out.
In the case of Advance Directives to refuse life sustaining treatment, the Directive has to comply with certain formalities. If this includes a refusal of specific treatment or a refusal of treatment in stated circumstances which will, inevitably lead to death, the Directive must be in writing, enclose a statement confirming your understanding that you will die as a result of your refusal and be signed and witnessed.