The word ‘custom’ needs clarification, markup is allowed in much the same manner as would be in posting comments to a blog post there are a series of allowed tags – but not many – such as <blockquote>, <code>, <p>
These tags should not present an issue, they are simple markup tags (most users will be unaware they can use them)
If you have something else being allowed then you must specify clearly what the conditions are that you refer to i.e What tags?! Are you running plugins which are altering the default behaviour?