If you'd still benefit from a solution to the problem, just leave the questions open and don't worry about your accept rate. You currently have a accept rate of 88%. That's more than enough. Anything above 70% is actually considered excellent.
If you still need a solution, consider placing a small bounty to draw attention to your question.
If you don't need a solution any more (the problem fixed itself, etc.) and you think nobody else will ever encounter the same problem, you can delete your question. But doing this only to improve your accept rate is wrong (it's just a meaningless statistic).
Moreover, bulk-deleting question might you get question banned, so reserve deletion for truly obsolete questions.