In a situation where you have been invited to or have viewed an opportunity, but you are no longer interested in submitting a bid for that opportunity, you are welcome to notify the buyer directly that you are no longer interested in submitting but this normally isn't a requirement unless noted in the RFP documents.
If the project specifically asks for an Intent to Bid, you can select the "No" button to show that you aren't interested in bidding on the opportunity, as long as the Intent to Bid period is still active. The Intent to Bid can be found under the Submissions heading.
NOTE: If you've downloaded any of the bid documents for the opportunity you will receive notifications and Public Notices related to the opportunity, even if you've indicated that you don't intent to bid. At this time there isn't a way to turn these notifications off, so please feel free to ignore them!
If you do not see the screen above, then you do not have to express that you have No Intent to Bid, unless the buyer specifies that you should submit this answer elsewhere (e.g., by emailing them or uploading a file).