How do I delete my Transformice account?

You can't. Just abandon your account. But, you can change your passWord if you have an email account (when ingame, click on menu, then change passWord).

You should probably contact a moderator if you have any problems with random people using your account, other people bothering you (pressuring you with personal questions, being racist, etc. ) . But then again, that might not be your problem at all. c;