I need strict language?

Question

My supplier is delaying supply of spare parts of a device which I bought long back and wanna warn them that I will cease my company deals with them hereafter. Please help.

Answer 1

Please be advised that your delay in shipping the parts I purchased on (date) are causing an unnecessary and extreme hardship for my business.  

We have had a long and successful business partnership for a number of years, and I would hate to see it ruined over this situiation.  Please ship the items immediately, and provide proof of shipment to my attention within 24 hours or provide an immediate refund of my funds in the amount of ($whatever) within the same time period.

(I wouldn't say you are ceasing dealints with them now, or you may not get your parts/refund at all, as they have nothing to lose)