First of all, I would not refuse to sign the agreement, take the training, and then leave in less than 12 months and expect to not have to reimburse the company. I think that would be dishonest. While I wouldn't like being in that situation either, I think it's fair for the company to say, Hey, if we make this big investment in training you, we don't want you to just quit the day after you get back and go take all that skill and knowledge that we paid for to a competing company.
What options is the company giving you?
If they say, "Everyone is required to take this training, and you must sign this agreement", then you are in a bind. At that point your only really choices are to either quit your job before the training; take the training, quit, and then reimburse the company; or delay quitting. Which I would do would depend a lot on the circumstances. If the training is valuable in future jobs and/or will look good on a resume, and you have no reason to leave on a specific date, you just don't particularly like the job, then you might just take the training and delay quitting. If there is some reason why you really must quit in less than 12 months -- you have another job already lined up or this job is so awful that you will kill yourself if you have to stay or whatever -- then you just have to make a decision which option is least objectionable.
If not taking the training is a realistic option, then that is what I would do. Like @Rus925 said, if they just sent out an email or a memo that says, "Anybody interested in this training, here's how to sign up and, oh, you'll have to sign this agreement", you could just not reply to the memo. If there are many people in the company not taking the training for any number of reasons, it's quite possible that no one will ask why you didn't. If it's optional, it's likely that many will say, "I can't take that much time away from my regular work" or even "I'm just not particularly interested".
If you must explain, you don't have to reveal specific plans. You could say, "I'm just not prepared to make this commitment. Who knows what will happen in the next 12 months?" If you just started at this job, you might say, "Hey, I just started here. I don't know how things will turn out."
By the way, I had a job a few years back where the company would pay for employees to go to college, but with a similar requirement that if you quit in less than -- I forget whether it was a year or how long -- that you had to repay the money. I took a few classes toward a master's degree under that program but then quit. I never paid any money back, I forget if I waited long enough or if they just didn't bother to enforce the policy.
A similar experience that I have now had at two companies is that after I took the job and had been working there for some time, they came up with new agreements that they wanted me to sign.
In one case they wanted me to sign a non-compete agreement that said that if I quit, I would not get any job involving computers -- I'm a software developer -- for one year. Well I wasn't going to sign that. I had no plans to quit at the time, but the odds were that sooner or later I would, and signing that would mean that I would basically have to agree to be unemployed for a year. Well, I suppose I could flip burgers at a fast food place. In that case I put my foot down and said that I'd agree not to go to work for a competing company, but I wouldn't agree to be unemployed for a year. The company agreed to change the contract, basically changed it from "... job involving computer technology of any kind whatsoever ..." to "... job involving computer technology that competes with ..." (The actual words in the contract were "of any kind whatsoever".) If they had insisted on the original wording, I don't know what choice I would have had but to quit. Better to be unemployed for a few weeks while I searched for another job than for a year.
Another one was a job where they suddenly demanded that all employees sign an agreement saying that any inventions they made or products they developed while working for the company belonged to the company. So if you spent your evenings and weekends developing some new product at home, using your own tools and equipment, not involving the company in any way ... it belonged to them. The agreement even specifically said "whether or not using company property or equipment". I and many other employees didn't want to sign that. Lots of people have dreams of starting their own business or at least some side business. In that case, I just never signed it. My boss came to me once and said that he needed to get my signed copy of the agreement. I just mumbled some excuse and never gave it to him, and nobody brought it up again. If the company had pressed, had said sign this or you're fired ... well, in that case the agreement was only for while you were working at the company, so I probably would have signed and then promptly started looking for another job.