Blocking certain websites from Sales Insight Web Activity Reports

It sounds like you are talking about the feed in SFDC, but the title indicated the reports in Marketo.

For the feeds in SFDC, reps will see leads in their ownership. To exclude people that are confusing them or are not responsive, you or they will need to change the ownership of the lead to a lead queue or another owner (maybe the marketo dedicated user?). That lead will then drop off the feed for that rep.

Here's what I would do for the Activity Reports in Marketo:

1. Create a list in your lead database of these leads. (It can be Smart or Static, just as long as the list contains or will contain people you want to exclude.)
2. On the Smart List tab of the Web Activity Report, add a filter of Member of List or Memeber of Smart List (depending on what list type you chose in step 1).
3. Change the drop down to "not in" and enter the name of the list you created.

Sorry for the confusion Joe - actually, the rep is searching the anonymous web activity in Marketo Sales Insight and prospecting from there. Is there a way to block companies from appearing on anonymous web activity report in Marketo Sales Insight within Salesforce? 

Jenny, I do not this functionality exists today.