Cbjeens

Again, I recommend you withdraw your petition -  we know that you can decide not to marry him once he is in US but he can overstay his visa like many illegal immigrants and this may come back to you.

First off, please provide a little more specific information. Where your paperwork is in the process will determine what steps you have to do to stop the process. 
 
Your visa petition was approved, NOA2? You are now waiting for the approved visa petition to go to NVC? You have a DOS visa casefile number? Is the approved visa petition  yet at the interviewing embassy/consulate?
 
To avoid any potential future repercussions, I would suggest that you, the petitioner, craft a visa withdrawal letter, short and sweet.  Send a copy to the interviewing embassy/consulate and a copy to the servicing USCIS service center. In it I would reference your DOS casefile number,  your USCIS reference number, and the name of your ex-beneficiary. I would include a brief statement explaining your current relationship situation, and your request for withdrawal from further consideration or advancement of this visa application. Do not file a Form I-134.  This withdrawal is final, and cannot be changed, reinstated, or appealed. In your letter to USCIS, I would likely include a copy of your I-129F visa petition approval letter (NOA2) for reference. I would likely email the letter to the interviewing embassy/consulate, and mail the letter with attachments to the USCIS service center first class, return receipt requested.
 
Best wishes on your future immigration journey.

You can stop the process prior to issuance of the visa by contacting USCIS, NVC, and the consulate.  Once a visa is issued, he can possibly get to the US....CBP can deny entry if they are notified prior to his entry.  ..If he gets inside the US, he can not remain legally if he doesn't marry YOU within 90 days after entry.

He won’t be able to come to the US under the k1 visa if you’re not going to marry him. He would have to leave after the 90 days visa expiry date. If you’re having doubts and you're having seconds thoughts about calling it off, do it before you get married because it’ll be way harder to “get rid” of that after getting married. 
 

If you decided so, you can withdraw your petition by writing a letter with your signature to USCIS and NVC/Consulate (if it is at NVC)/Consulate). If you have doubt in a relationship, there must be reason, investigate and reason it with logic to avoid regret and...asking how to get your future-husband deported. If you  withdraw your application and/or you do not submit I-134, then he will not be able to come to US. However, to avoid any future complication, withdraw your petition and notify USCIS/NVC/Consulate. Do not simply do nothing.