it sounds as though your assignment involves writing an email in the voice of a manager trying to remind employees not to use office supplies for personal use at home. The main rhetorical issue here is that you you don't want to run over your budget for supplies, but also you don't want to alienate your workers.
Although you may wish to cite company policy, you are most likely to get cooperation if you make your employees active stakeholders in the issue. Rather than simply issue an edict, you can explain what your current budget is for office supplies and that the department is running over budget. Next you could explain that you really do not want to inconvenience everyone by keeping supplies in a locked cabinet and having people fill out requisition forms every time they need a pen or paper clip.
In order to avoid this outcome, which no one wants, you should ask your employees to make an effort to reduce their use of office supplies, especially for use outside the office. By framing your email as a request for help in avoiding something that will inconvenience your employees, you are likely to get a better response than simply by chastising them for misusing supplies.