sounds like you have a loopback on the scart lead. you'd need to snip the video and audio feeds at one end so that the signals only go one way.
how have you got it connected? try this:
-vcr to tv aerial in via Rf cable, tune in tv to the vcr.
-set top box to vcr via scart.
-aerial to STB.
a quick test would be to connect the STB to the VCR with RCA phono cables (red /white/yellow) using a SCART to RCA adaptor plug(s) like this:
http://cc-tv.club24.co.uk/Scart-RCA.JPG
Tthat way you only use the STB 's audio and video outputs.