It is unfortunate that HOA enforcement sometimes seems to single out people unfairly. It clearly does happen, and it sounds like it may have happened to you. However, if all you have so far is letters alleging violations, you can dispute the violation, but it is unlikely that you have a harrassment suit against the HOA. I have clients who are HOAs that get accused of discrimination, and I have clients that are individuals who feel the HOA has singled them out. It is rare, however, that a lawsuit is the way for an owner to address the situation unless you have money to burn. Until some other action is taken by the HOA, such as assessing fines against you and filing a lien on the public records, the letters are an annoyance but are probably not the basis of legal liability.
What do you do? First, write back to the association and advise them that you contest the violation claim. Many association documents have dispute procedures written into the documents and require the board to have a hearing on the violation if requested. If they are required to have a hearing and refuse, they may not be able to take any further action without incurring liability. If the association has a managment company, talk to the property manager. Property managers deal with these situations day after day, and the manager may be of assistance in stemming the flood of violation notices.
Next, if the association takes any formal action, such as filing a lien or a lawsuit, you probably need to speak to an attorney who can look at the association documents and advise you of your options. If you just get a letter from an attorney for the association, consider contacting the attorney directly, but also consider having your own attorney make the contact. Don't simply ignore the letter as that is most unlikely to be helpful to you. Every association document is different, and the rights and options available to the association are provided for in the legal documents, not generally in the state statutes. Without looking at your specific documents, noone can fully advise you on your possible courses of action.
Finally, try to make yourself the most reasonable person in the dispute. If you can do that, and if the issue gets before a judge, you are far more likely to get a favorable result.
Good luck resolving the problem.