Well if your ISP is advertising a default along with full routes, which is probably not likely, as this is overkill, then you could use an inbound prefix list or distribute list or route-map to match just the 0.0.0.0 network.
To make sure you are not used as transit, you must config an as-path access-list and enable it outbound to the neighbours, this will allow only locally generated routes, ie thost with no current AS path;
1. Use a prefix-list to allow only the default route (make sure that your provider sends a default route in the first place):
ip prefix-list DEF seq 5 permit 0.0.0.0/0
router bgp x
neighbor x.x.x.x prefix-list DEF in
2. Make sure to only advertise your local routes to both providers and never advertise routes received from one provider to the other, example:
ip prefix-list LOCAL seq 5 permit x.x.x.x/x
router bgp x
neighbor x.x.x.x prefix-list LOCAL out
 Lee, sorry for the cross post.
 Using an as-path access-list as Lee suggested is the smartest way of doing it, but i've seen some customers whom are more comfortable to see the exact routes they are permitting in the configuration, its your choice.
Yes you can use " neighbor x.x.x.x distribute-list x in" plus an ACL, but i generally prefer using prefix-lists with route-filtering as they are more scalable, flexible and less CPU intensive than ACLs when we are talking about a large list.
A wildcard of 0.0.0.0 = host, meaning that it must be an exact full 32 bit match (and it is the default in case you didn't state an explicit wildcard), and thus "access-list 1 permit host 0.0.0.0" = "access-list 1 permit 0.0.0.0 0.0.0.0" = "access-list 1 permit 0.0.0.0".
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