There are 2 options you can follow according to your preferences
1- You need to copy thecomponents/com_k2/css/k2.css file to your template's css folder and remove these rules from the copied file:
div.itemTagsBlock ul.itemTags li a:before {content:"#";}
div.catItemTagsBlock ul.catItemTags li a:before {content:"#";}
div.userItemTagsBlock ul.userItemTags li a:before {content:"#";}
div.latestItemTagsBlock ul.latestItemTags li a:before {content:"#";}
2- Another method is available if your template supports a custom.css file, you will add these rules to it:
div.itemTagsBlock ul.itemTags li a:before {content:none !important;}
div.catItemTagsBlock ul.catItemTags li a:before {content:none !important;}
div.userItemTagsBlock ul.userItemTags li a:before {content:none !important;}
div.latestItemTagsBlock ul.latestItemTags li a:before {content:none !important;}
If you have any questions or suggestions, feel free to comment on this post, or contact me directly via email or social media (on the above right corner of this page).