You can't stop her from claiming you. But,
If someone else claims you, as a dependent, inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming yourself, if appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you’ll receive a letter from the IRS, stating that your exemption was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the exemption, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.
Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest.
Your mother will only get $500 for claiming you. But you will get the $1800 stimulus, by claiming yourself, and maybe (unlikely)* more. Quite frankly, it's not only wrong for her to try to claim you and costly.
*With the tax law change, effective 2018, most dependents will get the same refund (not counting stimulus) whether they claim themselves or not. The personal exemption has been eliminated and the standard deduction increased. However, you only qualify for an education credit or deduction, if you are not a dependent.
