You are lucky to have that problem. While it is easy just walk away and leave the offer hanging, it is still better to inform the recruiter that you are accepting a position in another company. It is better to do it in person than over the phone. Never ever decline an offer through text.
Even if you are absolutely sure that you will not be working for the company in the future, it is a good practice to leave the best impression when declining a job offer. Show appreciation for their interest in making you a part of the company. Also thank the recruiter for their time and effort. Never say anything negative about the company, no matter how true it is, when declining the offer. You just don’t know what the future holds, so don’t burn bridges this early.