Delete table rows where column text does not begin with "x"

Edit 4/16/20: Either this is really hard or the original title wasn't flashy enough. Made some editorial changes to try and simpify the question.

Greetings,

First, I know. I know there are a million of these types of questions on here. Believe me when I say I've spent an hour searching and trying different things before asking a question that I'm sure will have an easy answer. But I either haven't understood the principles, or haven't stumbled on the exact right method yet.

I have a table that is 3658x1. The first column has hex values that look like this:

15:00:51:c3:1f:fb:6b:c1:a3:00:00:01:01:08:0f:3b:31:11.... you get the idea

I want delete all rows that do not start with hex 15, as shown in the example above.

Thanks!

- Roger