1) The former company can certainly try to woo the clients back, or else refrain from supporting you in your new endeavor---that is, as long as there's no agreement or contract requiring them to support you. Otherwise, they can rescind their offer to help.2) However, unless you either signed (and are violating) some noncompetition agreement and/or are using trade secrets, a client/customer list (see below), or representing yourself as still affiliated with them, they can't stop you from working.3) You might not have a right to use their client or customer list without permission--that's company property--though nothing stops you from marketing yourself generally and if any old clients or customers come to you, in the absence of a noncompete, you could work with them.4) If the old company had specifically encourage you to contact  this client and it's provable, then even if you couldn't contact other clients, it would difficult for them to now say you can't work with this particular one. You should, however, bring any correspondence, memos, agreements, etc. to an employment law attorney who can evaluate this specific situation in detail for  you.