If you are creating the whole email in html, try adding this to your head section style tag:
#outlook a {padding:0;}
If that doesn't work you could try adding !important
If it is just an email signature, and not a full html document, there is no way to put in a <style>
tag declaration there. This is the only way to overwrite a <p>
tag that doesn't exist yet.
Best way in this case is to just try to design around it.