Can an employer force an employee to take the health insurance they are offered?

no, no employer can force you to sign up for medical benefits. If you are covered by a spouse, for example, there may be no need for you to sign up for your company's insurance program.
It may be required for you to sign a waiver stating you are refusing the insurance offered by your employer. This is usually accompanied by a statement stating that you are fully insurable within your first 30 days of employment. If you choose to participate in the insurance program after your inital 30 day employment period, you may be subject to a physical and / or other medical tests.  
Actually the previous answer that somebody gave is incorrect. If the employer is paying 100% of the premium cost they not only can force you to be on the plan, but they must. Under federal law they would be discriminating against you if they did not. You cannot waive off the policy. However, if you are paying ANY portion of the premium, you have the right to waive off the policy.


 
Actually, it's worse. Both of the previous answers are wrong, according to the Department of Labor. Their representative states that there is no law that bars a company from making its employees accept the health coverage options it offers-- even if the employer pays none of the premium. I repeatedly ask for a law that speaks to this question, and they repeatedly said there is no law. They related it to practices like companies requiring that employees have (and pay for) uniforms. They can make this a requirement of employment.

So Yes, an employer can force its employees to accept the health coverage and pay out of pocket. They may offer waivers, but they are not required to by law.