BGP doesn’t really work quite how you want it to, but to get a good approximation:
you should have a filter on your BGP sessions limiting what prefixes you send to your suppliers (create filter under /routing/filter and then apply it to your BGP session as an “out-filter”
you can set a lower local-preference on the session to your “backup” suppliers to tell your routers to “demote” any routes learnt from this supplier - this should mean that the routes from your primary provider will take precedence
prepend your AS a couple of times to the “backup” provider - this will reduce the amount of traffic you get into your network from this provider (but not down to zero traffic)
your backup provider may provide some communities you can tag your prefixes with to set a lower local-pref on /their/ network