You can't delete your own topics. As suggested above, all you can do is to edit their contents to something like "delete" or anything else then move them to Archive using the "move topic" button at bottom left.
Also make sure to check your PMs in case whoever got access to your account sent unsolicited messages to other members. If someone reports such pms it may result in temp-banning your account.

Besides changing your password, try to figure out how your account got hacked. Changing the password while the device is still compromised won't change much.

Hi there,

You can see if someone has accessed your account using this method : go to your profile, go to "account related settings" , then under limit IP, there's a link to "Your logged IPs for the last 30 days". If indeed it was someone else, you will be able to see a change in IP used. Surely, then you will have a concrete evidence and mods will be able to help you better. You might wanna also edit your posts which you wanna delete.

Why dont you just edit them to remove content and move to archive?