High GA3 concentration is known to inhibit flowering of roses, see for example:
There is a chemical that is a known gibberellic acid biosynthesis inhibitor. It is prohexadione-calcium (ProCa). A 2005 Ph.D. thesis that investigated this compound is available for free download at: Http://library.wur.nl/wda/dissertations/dis3808.pdf
The author studied strawberries but the thesis may give some information at least worth trying on roses. I particularlly recommend reading the 2 chapters starting on page 81 and on page 89.
The following link gives the available commercial products: