Is it permissible to dispose of surplus funds in my bank account, after contacting the bank three times and informing them that the amount is not mine, and they have not responded to me yet?

If you are certain that the amount does not belong to you and its reason is not apparent, do not dispose of it. Instead, contact the bank to ascertain its source. If it was transferred to you by mistake, return it to its owner. If you cannot identify its source after searching and a year has passed, you may use it, provided you guarantee it to its owner should they appear, treating it like a lost and found item (Luqata).