Hi Claude,
Assuming this is situtation already created in Marketo.
" Anyone who fills out a contact us form, immediately would receive Thank you email as trigger"
And to remove the bounced or spam emails from further receiving Auto response email, you can create this type of campaign.
Smart List : ALL filter
1 Anyone who fills out Contact us form
2. Email bounced is true
Flow :
1. Remove from the program or campign - you have already created for Thank you email.
To keep clean data, you can add one more action in flow.
2.Delete Lead from Marketo and SFDC
Else as Josh suggested, many company helps in validating emails.
Hope this solves your query.
Thank you