I tried to disable this in the admin area to no avail. I then wrote a style rule for the custom template to override the vertical margin associated with said tags which works fine when I test in the admin area but fails in production when emails are sent. Marketo, you suck!
Yes this is annoying. Tip: you can use <div> tags which won't have default margins like <p>
I agree this isn't really a helpful default that Marketo sets out-of-the-box. You can adjust this setting for both LPs and EMs under the Email section in the Admin menu.
Admin > Email > Edit Text Editor Settings > change both dropdowns to "none".
By default, the LPs are set to have a <div> around the content and the EMs are set to have a <p> around the content.
Another thing that I've noticed is that you'll occasionally get a "not-so helpful" hand with <br> tags being injected into empty divs inside an editable area >> EX. <div class="spacer"></div> becomes <div class="spacer"><br></div>. To work around this, I'll usually put an empty comment tag inside the empty element >> EX. <div class="spacer"><!-- --></div> and this seems to prevent the <br> from getting added.
Hope this is helpful, let me know if you run into any issues changing this.